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This allows XHRs from the background page to file:/// URLs to succeed. XHRs from
content scripts still fail, since they're running in a regular renderer process,
and there is currently no mechanism for only whitelisting the isolated world
within it for the file: scheme.

Also re-enable ExtensionApiTest.CrossOriginXHRContentScript since it doesn't
appear to have flaked recently.

BUG=104547,96725
R=aa@chromium.org


Review URL: http://codereview.chromium.org/8587052

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@110614 0039d316-1c4b-4281-b951-d872f2087c98