SyncFS v2: Fix missing flag handling

* Keep missing==true for inactive tracker at the end of RemoteToLocalSyncer
* Check missing inconsistency only for active trackers in ShouldKeepDirty

BUG=240165
R=tzik@chromium.org

Review URL: https://codereview.chromium.org/110413004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240657 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240657