Schedule layout tree update for dirty tree scopes.

In preparation for async stylesheet update, schedule a layout tree
update when marking tree scopes dirty for active style sheets. This is
necessary to trigger a beginFrame which will in turn call
updateActiveStyle as part of the lifecycle update.

R=meade@chromium.org
BUG=567021

Review-Url: https://codereview.chromium.org/2547883002
Cr-Commit-Position: refs/heads/master@{#436248}
1 file changed