[SyncFS] Split LargestChangeID to LargestFetchedChangeID and LargestKnownChangeID

BUG=240165

Review URL: https://codereview.chromium.org/66193003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234189 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234189