Added more complex profiling metadata.

This changes the profiling output from:

```
_category_                              _mean (ms)_     _median (ms)_   _min (ms)_      _max (ms)_      _bucket size_
CompositingCategory                :          5.3075          4.7763          1.4280         16.6432              12
LayoutPerformCategory              :        284.9125        202.3233        184.6329        967.9776              19
+ LayoutStyleRecalcCategory        :        242.7735        161.5772        146.5625        922.5363              19
+ LayoutDamagePropagateCategory    :          0.6145          0.5888          0.5176          0.8663              19
+ LayoutMainCategory               :         12.3587         11.3791         10.1172         20.1418              19
| + LayoutParallelWarmupCategory   :          0.0008          0.0008          0.0006          0.0014              38
+ LayoutDispListBuildCategory      :         25.6660         23.7513         21.2865         46.7053              19
RenderingDrawingCategory           :         19.6144         19.5144         11.0669         79.0099             150
RenderingCategory                  :        333.6637        349.5627          0.0053        423.6390              13
```

to:

```
_category_                          _incremental?_ _iframe?_             _url_              _mean (ms)_     _median (ms)_   _min (ms)_      _max (ms)_      _bucket size_
CompositingCategory                     N/A          N/A                  N/A                        5.7452          5.0217          1.6569         19.6917              11
LayoutPerformCategory                incr reflow    page     http://cnn.com/                       283.5427        201.8654        184.0476       1116.7777              18
LayoutPerformCategory               first reflow    page     http://www.cnn.com/                   325.5460        325.5460        325.5460        325.5460               1
+ LayoutStyleRecalcCategory          incr reflow    page     http://cnn.com/                       239.8541        163.0796        144.7337       1080.4143              18
+ LayoutStyleRecalcCategory         first reflow    page     http://www.cnn.com/                   261.4809        261.4809        261.4809        261.4809               1
+ LayoutDamagePropagateCategory      incr reflow    page     http://cnn.com/                         0.6259          0.6168          0.5312          0.8752              18
+ LayoutDamagePropagateCategory     first reflow    page     http://www.cnn.com/                     0.6078          0.6078          0.6078          0.6078               1
+ LayoutMainCategory                 incr reflow    page     http://cnn.com/                        12.3509         11.9542         10.9876         14.1981              18
+ LayoutMainCategory                first reflow    page     http://www.cnn.com/                    11.3846         11.3846         11.3846         11.3846               1
| + LayoutParallelWarmupCategory     incr reflow    page     http://cnn.com/                         0.0009          0.0009          0.0006          0.0022              36
| + LayoutParallelWarmupCategory    first reflow    page     http://www.cnn.com/                     0.0009          0.0011          0.0007          0.0011               2
+ LayoutDispListBuildCategory        incr reflow    page     http://cnn.com/                        28.0712         24.4887         22.8510         73.7452              18
+ LayoutDispListBuildCategory       first reflow    page     http://www.cnn.com/                    50.3867         50.3867         50.3867         50.3867               1
RenderingDrawingCategory                N/A          N/A                  N/A                       22.2310         21.1877         13.1922         74.4110             138
RenderingCategory                       N/A          N/A                  N/A                      372.2654        390.7910          0.0087        489.5884              12
```

This patch will not currently build without a checkout of
`https://github.com/cgaebel/rust-url:cgaebel-servo-backport` listed as an
override in your ~/.cargo/config.

This is blocked on https://github.com/servo/rust-url/pull/29, however, is ready
for review now.