Introduce setNeedsActiveStyleUpdate for adding/removing stylesheets.

Remove the add/remove/modify methods which did not have different
implementations anyway. The plan is to let the async active stylesheet
update detect which StyleSheetContents have been added and which have
been removed and invalidate style and caches accordingly.

I've started to write up the plan here:


Review URL:

Cr-Commit-Position: refs/heads/master@{#384008}
9 files changed