[SyncFS] Implement LargestKnownChangeID handling

BUG=240165
R=kinuko@chromium.org, nhiroki@chromium.org

Review URL: https://codereview.chromium.org/63243004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235611 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235611