[SyncFS] Run ConflictResolver only when it is needed.

Introduce |should_check_conflict_| flag to SyncEngine and run ConflictResolver only if either:
 - RemoteToLocalSyncer,
 - LocalToRemoteSyncer,
 - ListChangesTask
 - ConlfictResolver
did non-noop task.

BUG=240165,326423

Review URL: https://codereview.chromium.org/98043004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239211 0039d316-1c4b-4281-b951-d872f2087c98
Files.app: Update availability of OK button by using input events.

Previously the availability is updated by handling keydown events.  But when the
events are published, the input value has not updated yet.  This CL turns the
events handled to update the availability into input events.

BUG=326094
TEST=Open save file dialog, input and delete text in the filename input.

Review URL: https://codereview.chromium.org/108013003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239212 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239212