Add partial load tests for PNG images

Add partial load tests for PNG images
https://bugs.webkit.org/show_bug.cgi?id=95707

Reviewed by Adam Barth.

Partial load test: receive a partial number of image bytes and stall forever. The
partial image should be decoded and drawn and the blue <img> background should be
visible.

Progressive load test: receive a partial number of image bytes, stall for 1 second
and then continue to receive all the image bytes. The entire test image should be
decoded and drawn and the blue <img> background should be visible.

* fast/images/resources/dice.png: Added: test image has alpha transparency.
* http/tests/images/png-partial-load-expected.png: Added.
* http/tests/images/png-partial-load-expected.txt: Added.
* http/tests/images/png-partial-load.html: Added.
* http/tests/images/png-progressive-load-expected.png: Added.
* http/tests/images/png-progressive-load-expected.txt: Added.
* http/tests/images/png-progressive-load.html: Added.
* platform/chromium/http/tests/images/png-partial-load-expected.png: Added.
* platform/mac/http/tests/images/png-partial-load-expected.png: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@128961 268f45cc-cd09-0410-ab3c-d52691b4dbfc