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

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.

Committed: https://code.google.com/p/grit-i18n/source/detail?r=124

[Jói, 2013-06-08 14:20:47]

Committed patchset #1 manually as r124 (presubmit successful).