Handle Google Forms documents in Files.app.

BUG=162575
TEST=Manually create a form in drive.google.com and see it appears with an icon in Files.app.
R=satorux@chromium.org, yoshiki@chromium.org

Review URL: https://codereview.chromium.org/28323002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230058 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230058