[SyncFS] Implement conflict detection functions to MetadataDatabase

BUG=240165
NOTRY=true

Review URL: https://codereview.chromium.org/94233002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237860 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237860