Allow accessing data URL resource using CORS. This is required
to allow XMLHttpRequest to access data URL based on CORS so that
the resource is still handled as cross-origin.

See also Chromium side change
https://codereview.chromium.org/54233002/ which adds a CORS header
to responses to data URL loading request.

Also add layout tests to check that XMLHttpRequest can load data URLs

BUG=308768

Review URL: https://codereview.chromium.org/54173002

git-svn-id: svn://svn.chromium.org/blink/trunk@175037 bbb929c8-8fbe-4397-9dbb-9b2b20218538