SyncFS v2: Skip Prepare for remote folder update for existing folder

Same change as in https://codereview.chromium.org/102453004/, but with fixed tests

BUG=none
TEST=manual

Review URL: https://codereview.chromium.org/115213002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240712 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240712