[SyncFS] Add stub impl of LocalToRemoteSyncer

BUG=240165

Review URL: https://codereview.chromium.org/75103003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235982 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235982