Add separate trace events for recalcStyle and rebuildLayoutTree.

When style recalc and layout tree building is fully separated, it will
be valuable to measure them separately as well.

R=nainar@chromium.org,esprehn@chromium.org
BUG=595137

Review-Url: https://codereview.chromium.org/2779573002
Cr-Commit-Position: refs/heads/master@{#460027}
1 file changed