Convert some pixel tests in fast/canvas into ref tests.

This change removes many pixel baselines that were trivial enough
to be expressed as reference html. Some of the tests were slightly
modified to make them work as ref tests.

R=senorblanco@chromium.org
BUG=349536

Review URL: https://codereview.chromium.org/187393007

git-svn-id: svn://svn.chromium.org/blink/trunk@168540 bbb929c8-8fbe-4397-9dbb-9b2b20218538