Make ImageFrame be transparent by default

Make ImageFrame be transparent by default

Previously, ImageFrame was initialized to not have alpha, which means
that it would be considered to be opaque by other parts of the system.
Instead, initialize it to think it has alpha when it isn't known (which
the rest of the class also does).

This fix prevents flakiness in painting where loading caused raciness
with whether or not a transparent background image would cause a
background color to not get painted because of different values being
returned from backgroundHasOpaqueTopLayer.

BUG=442991
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=187404

[enne (OOO), 2014-12-17 19:55:00]

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

[pdr., 2014-12-17 20:07:45]

The CQ bit was checked by pdr@chromium.org

[pdr., 2014-12-17 20:07:46]

lgtm

[commit-bot: I haz the power, 2014-12-17 20:07:57]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/814573003/1

[commit-bot: I haz the power, 2014-12-17 21:27:44]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=187404