Consider the origin when computing extension permissions

      This patch teaches the extension system to use the document's origin when
computing extension permissions. Ideally, we'd use only the document's origin,
but because app extents don't cover entire origins, we need to also consider
the document's URL.

BUG=103630
Review URL: http://codereview.chromium.org/8659009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@112655 0039d316-1c4b-4281-b951-d872f2087c98