Extension syncing: Introduce a NeedsSync pref
that indicates the extension has local changes that still need to be synced.
It's set when something changes before sync is ready, and cleared once the extension state has been synced.
This should handle all conflicts between sync and local state reasonably well, and as a bonus allows us to get rid of the (weird and not-really-working) PendingEnables class.
BUG=509990
Review URL: https://codereview.chromium.org/
1240573012
Cr-Commit-Position: refs/heads/master@{#339651}