SyncFS: Active tracker for missing file should be marked as missing

Current RemoteToLocalSyncer is not able to handle newly added tracker for new
file/folder properly like this:

1) A tracker is added as dirty & inactive
2) The inactive tracker is not handled by RTLSyncer::ResolveRemoteChange()
3) MetadataDatabase::UpdateTracker() activates the tracker and fills in
its synced_details
4) The tracker is not handled by ResolveRemoteChange() as new file/folder
case since it has synced_details

This change treats newly added tracker like this:

1), 2) Ditto.
3) UpdateTracker() activates the tracker, marks it as missing and fills in
its synced_details 
4) Thanks to the missing flag, the tracker is handled by ResolveRemoteChange()
as new file/folder case.

NOTE:
- Active tracker should always have synced_details due to 3)


BUG=240165
TEST=n/a (to be added in a separate CL)

Review URL: https://codereview.chromium.org/79153004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236800 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236800