Revert 112178 - [Sync] Make syncer commands avoid posting tasks on threads with no work to do

Revert 112178 - [Sync] Make syncer commands avoid posting tasks on threads with no work to do

Add abstract GetGroupsToChange() method to ModelChangingSyncerCommand.
Use that to figure out which worker threads to post work on (instead of
posting on all of them).

Implement GetGroupsToChange() for each ModelChangingSyncerCommand.

Add GetEnabledGroups() and GetEnabledGroupsWithConflicts() functions to SyncSession.

Key unapplied updates index by type for ApplyUpdatesCommand.

Make the abstract methods of ModelChangingSyncerCommand protected.

BUG=97832
TEST=


Review URL: http://codereview.chromium.org/8637006

TBR=akalin@chromium.org
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=112318