Move srcset's canvas tests to ref tests

I've replaced srcset's canvas tests from pixel tests to ref tests & bullet-proofed the ref tests to fail in case that the images are missing.

I've also deleted tons of platform specific expected results in the process.

BUG=

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=163379

Review URL: https://codereview.chromium.org/109613002

git-svn-id: svn://svn.chromium.org/blink/trunk@163381 bbb929c8-8fbe-4397-9dbb-9b2b20218538