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

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
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=175164

[Justin Novosad, 2014-05-30 17:49:39]

PTAL

[Stephen White, 2014-05-30 18:33:20]

LGTM

[Justin Novosad, 2014-05-30 18:46:21]

The CQ bit was checked by junov@chromium.org

[commit-bot: I haz the power, 2014-05-30 18:46:59]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/junov@chromium.org/302083006/1

[commit-bot: I haz the power, 2014-05-30 19:23:19]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  blink_presubmit on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/blink_presubmit/builds/6548)
  linux_blink_dbg on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/linux_blink_dbg/builds/1...)
  linux_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/linux_blink_rel/builds/1...)
  linux_gpu on tryserver.chromium.gpu
(http://build.chromium.org/p/tryserver.chromium.gpu/builders/linux_gpu/builds/...)

[commit-bot: I haz the power, 2014-05-30 22:52:19]

Change committed as 175164