Files.app: Remove support for chrome://files.

Files.app has supported chrome://files URL, but it is no longer needed.
This CL removes the support and the related tests.

BUG=288447
TEST=manually

Review URL: https://codereview.chromium.org/27510009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229598 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229598