Cleanup the file path normalization extension_protocol.
Cleanup the file path normalization extension_protocol.

I had hackily put in forward slashes, but now that there's a
normalization method for filepath, I'm using it.

Review URL: http://codereview.chromium.org/2833032

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@50958 0039d316-1c4b-4281-b951-d872f2087c98

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