Normalize image/x-png mime type to the standard one

On Windows, Python's mimetypes.guess_type() would return image/x-png
type for PNG files as its default mappings are overriden by
a Windows specific mappings read from the registry.

Using image/x-png for such resources would trigger a warning when
such inlined resource is loaded in the browser (Blink engine at least).

Create a helper function that normalizes such known, but unwanted
mime types to standard ones.

Review URL: https://codereview.chromium.org/15981005

git-svn-id: http://grit-i18n.googlecode.com/svn/trunk@124 7262f16d-afe8-6277-6482-052fa10e57b1