Convert fast/repaint/background-scaling.html to a reftest

Convert fast/repaint/background-scaling.html to a reftest

Originally the test expects full repaint of the viewport on page scale
changes. However, the repaint is unnecessary because we handle page
scale in the compositor. My later change will optimize this.

Convert it to a reftest to ensure the visual result is correct.

BUG=258219
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=173658

[abarth-chromium, 2014-05-08 12:56:19]

lgtm

[commit-bot: I haz the power, 2014-05-08 12:56:58]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/wangxianzhu@chromium.org/267333008/20001

[commit-bot: I haz the power, 2014-05-08 14:08:59]

FYI, CQ is re-trying this CL (attempt #1).
Please consider checking whether the failures are real,
and report flakes to chrome-troopers@google.com.
The failing builders are:
  mac_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/mac_blink_rel/builds/6182)

[commit-bot: I haz the power, 2014-05-08 14:45:37]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-05-08 14:45:37]

Try jobs failed on following builders:
  mac_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/mac_blink_rel/builds/6194)

[Xianzhu, 2014-05-08 16:28:43]

The CQ bit was checked by wangxianzhu@chromium.org

[commit-bot: I haz the power, 2014-05-08 16:29:52]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/wangxianzhu@chromium.org/267333008/40001

[commit-bot: I haz the power, 2014-05-08 17:45:05]

Change committed as 173658