Fixed layout test fast/canvas/canvas-bg.html to work with accelerated canvas

This patch removes the semi-transparent color used int the test, which
was irrelevant to the feature being testes and was causing the test
to fail due to rounding errors in compositing.

BUG=368008

Review URL: https://codereview.chromium.org/302083006

git-svn-id: svn://svn.chromium.org/blink/trunk@175164 bbb929c8-8fbe-4397-9dbb-9b2b20218538