Introduce markAllTreeScopesDirty.

When we need to recollect active stylesheets for all scopes, have an
explicit markAllTreeScopesDirty method instead of relying on
FullStyleUpdate which will go away for async active stylesheet updates.

This CL does not contain functional changes.

R=meade@chromium.org
BUG=567021

Review-Url: https://codereview.chromium.org/2534863002
Cr-Commit-Position: refs/heads/master@{#434940}
3 files changed