Invalidate background obscuration after loading background images

Invalidate background obscuration after loading background images

This patch fixes a bug where a background would be considered obscured
during loading but would not get reexamined for the background
obscuration bit after the image loaded. Other loading codepaths
(e.g., <images>) had this case covered but background images were
missing this check.

BUG=422272
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=201804

[pdr., 2015-09-04 02:22:34]

The CQ bit was checked by pdr@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-09-04 02:22:46]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1312943008/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1312943008/1

[pdr., 2015-09-04 03:17:21]

pdr@chromium.org changed reviewers:
+ chrishtr@chromium.org

[commit-bot: I haz the power, 2015-09-04 03:19:58]

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

[commit-bot: I haz the power, 2015-09-04 03:19:59]

Dry run: This issue passed the CQ dry run.

[chrishtr, 2015-09-04 17:02:17]

The CQ bit was checked by chrishtr@chromium.org

[chrishtr, 2015-09-04 17:02:17]

lgtm

[commit-bot: I haz the power, 2015-09-04 17:02:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1312943008/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1312943008/1

[commit-bot: I haz the power, 2015-09-04 17:08:27]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201804