Allow "chrome-extension://" to be used as web app url.
Allow "chrome-extension://" to be used as web app url.

BUG=29201
TEST=Verify a web app shortcut for "chrome-extension://" url could be created.

Review URL: http://codereview.chromium.org/2193002

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@48294 0039d316-1c4b-4281-b951-d872f2087c98

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