Deprecate ScopedExternalFileSystem

Before migrating from webkit/ to content/ we'll need to make one public
interface per one file, which means ScopedExternalFileSystem needs to be
separated out from external_mount_points.h -- but it turned out that
it's currently used only by one file, so I'm rather deprecating it.

BUG=265769
TEST=none

Review URL: https://codereview.chromium.org/26455002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227514 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227514