Add image loading spec related tests

Added the following layout tests:
* Setting order of crossOrigin and src attributes in JS doesn't change outgoing request
* Setting order of srcset and src attributes in JS doesn't change fetched resources
* Setting innerHTML with an <img> inside triggers a resource load
* Setting src of a cached image exposes its width immediately

BUG=

Review URL: https://codereview.chromium.org/136693020

git-svn-id: svn://svn.chromium.org/blink/trunk@166657 bbb929c8-8fbe-4397-9dbb-9b2b20218538