Add parsing of `overflow: scroll` and `auto`.

Elements with overflow scroll/auto are not yet scrollable, but they will be
clipped correctly (like `overflow: hidden`).
Remove unused Fragment::needs_clip method
Add a FIXME comment for #2742
Merge pull request #2740 from mbrubeck/scroll-auto

Basic parsing of `overflow: scroll` and `auto`.
Run render task on OS thread to avoid Skia memory corruption.

When Skia is running on a green task, it can sometimes be
rescheduled onto a different OS thread. This creates problems with
the memory pool implementation in Skia, which uses TLS for
storing the per thread memory pool information. Free'ing a
pool allocation from a different OS thread than it was
allocated on can cause heap corruption.

FIXME: When we update rustc, switch the task failure
message to using the new NativeTaskBuilder interface so that
it can use the same send_on_failure function as green tasks.
Merge pull request #2743 from glennw/skia-threading

Run render task on OS thread to avoid Skia memory corruption.