Use opaque black not transparent black as imagefilter GM background.

Use opaque black not transparent black as imagefilter GM background.

For some reason, GM's are now being composited against white
in some modes, where the alpha used to be ignored (or composited
against black, I'm not sure which). At any rate, it doesn't
make much sense to have alpha in the result anyway, so let's clear to
opaque black instead of transparent black and avoid the problem.

This is a trial balloon for bitmapsource and pictureimagefilter.
If all goes well, I'll make this change more widely.

R=scroggo
BUG=skia:3319
Committed: https://skia.googlesource.com/skia/+/aad8b27ec6ff93b64f458be78a3eff7e91b10e15

[Stephen White, 2015-04-08 17:56:57]

scroggo@: PTAL. Thanks!

[scroggo, 2015-04-08 18:46:18]

LGTM. Thanks!

> For some reason, GM's are now being composited against
> white in some modes, where the alpha used to be ignored
> (or composited against black, I'm not sure which).

I *think* the issue is not about changing the background GMs are drawn to, but
rather how we compare GM results to determine whether they are correct.

These GMs clear(ed) to transparent, resulting in a different result for 8888 and
565 - black in 565, which does not have transparency, and white in 8888.

In the old world, each GM had a blessed image for each config. So the blessed
565 had a black background, while the blessed 8888 had a white background. If a
CL changed them, they would be rebaselined separately, maintaining their
differences.

In the new world, we compare 8888 to 565 in gold.skia.org. Here we see the
differences.

(Maybe we also changed something about the destination? Not that I'm aware of
though.)

[Stephen White, 2015-04-08 18:47:57]

The CQ bit was checked by senorblanco@chromium.org

[commit-bot: I haz the power, 2015-04-08 18:48:24]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1074513002/40001

[commit-bot: I haz the power, 2015-04-08 19:36:14]

Committed patchset #3 (id:40001) as
https://skia.googlesource.com/skia/+/aad8b27ec6ff93b64f458be78a3eff7e91b10e15