Explicitly note that syncFileSystem doesn't support directory operations yet

BUG=318171

Review URL: https://codereview.chromium.org/66513009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235107 0039d316-1c4b-4281-b951-d872f2087c98
Blink roll 161970:161994

http://build.chromium.org/f/chromium/perf/dashboard/ui/changelog_blink.html?url=/trunk&range=161971:161994&mode=html
TBR=
BUG=

Review URL: https://codereview.chromium.org/72443003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235108 0039d316-1c4b-4281-b951-d872f2087c98
Update .DEPS.git

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235109 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235108
SVN changes up to revision 235109