FileAPI: Expose GrantCreateReadWriteFileSystem to ContentAPI


BUG=286240
TEST=n/a

Review URL: https://codereview.chromium.org/70133002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234780 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234780