LayoutTest Case for png images served with "image/x-png" mime type

The image/x-png is a non-standard mimetype, now replaced by image/png.
Adding support to be backward compatible and also match the behavior
of other browsers like Firefox. The patch adds the Layout TestCases.

BUG=25293

Review URL: https://codereview.chromium.org/270473002

git-svn-id: svn://svn.chromium.org/blink/trunk@173730 bbb929c8-8fbe-4397-9dbb-9b2b20218538