Add some more trace events related to layout

In investigating traces from the Silk benchmark, Ojan and I wished we had a bit
more resolution in the trace relating to layout. By adding the following trace
events, we were able to see that the time wasn't actually being spent sizing
elements but instead in other code below "layout."

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/132143008

git-svn-id: svn://svn.chromium.org/blink/trunk@164840 bbb929c8-8fbe-4397-9dbb-9b2b20218538