Remove bottom up root

recent chrome can work without a bottomUpRoot
it will generate it's own itself.
so better to stop wasting resources building it.
(this also reduce the .cpuprofile file size by nearly half)

sample cpuprofile: [get it here](https://gist.githubusercontent.com/arunoda/20bf8f5e4d7014ac3e56/raw/d5b9c21313a3d0863dd63786676ff3db519577b7/gistfile1.txt)
discussion: https://groups.google.com/forum/#!topic/v8-dev/Jms7ycmqcK0