[SyncFS] Implement RemoteToLocalSyncer::HandleContentUpdate

 * Reroute content update to HandleNewFile.

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

Review URL: https://codereview.chromium.org/71323002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235596 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235596