Setting preferred stylesheet simplified.

With active stylesheets being applied asynchronously, we no longer need
to avoid the synchronous stylesheet update during link processing. We
can just mark the treeScope dirty to trigger the preferred set to be
updated as part of updateActiveStyleSheets().

R=meade@chromium.org
BUG=567021

Review-Url: https://codereview.chromium.org/2552353003
Cr-Commit-Position: refs/heads/master@{#439115}
6 files changed