SyncFS: Fix when to call OnSyncIdle()

Without this fix demoted changes are not correctly promoted

BUG=330116, 240165
TEST=manual with logging (update - delete conflict)
R=nhiroki@chromium.org, tzik@chromium.org

Review URL: https://codereview.chromium.org/119793002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242059 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242059