Unflake layout test loader/data-uri-images-load-when-loading-images-automatically-disabled.html

Re-arranged the test to avoid using img.onload which doesn't seem to be
reliable enough.

Verified that the test still would detect the condition of data: uri images
being blocked by loadsImagesAutomatically.

BUG=387101,391388

Review URL: https://codereview.chromium.org/377513002

git-svn-id: svn://svn.chromium.org/blink/trunk@177594 bbb929c8-8fbe-4397-9dbb-9b2b20218538