[Android] Clear backing SkBitmap before rendering the overscroll effect

[Android] Clear backing SkBitmap before rendering the overscroll effect

Skia makes no guarantees about the contents of a newly allocated
SkBitmap. Consequently, it's possible that the bitmap allocated for the
overscroll effect isn't zeroed before the effect itself is painted.
Prevent related issues by always clearing the allocated SkBitmap's
contents (to fully transparent) before painting the effect.

BUG=408225
Committed: https://crrev.com/46c0ec38bed034b6fcaade3f23258bd796a93db6
Cr-Commit-Position: refs/heads/master@{#292280}

[jdduke (slow), 2014-08-27 22:02:46]

jdduke@chromium.org changed reviewers:
+ aelias@chromium.org

[jdduke (slow), 2014-08-27 22:02:46]

aelias@: PTAL.  I wasn't able to repro even with M38 official builds, and
askatte@ mentioned she had a hard time repro'ing as well.

[aelias_OOO_until_Jul13, 2014-08-27 22:16:11]

lgtm

I would still like to see a repro, although this change is definitely safe.

[jdduke (slow), 2014-08-27 22:25:38]

The CQ bit was checked by jdduke@chromium.org

[commit-bot: I haz the power, 2014-08-27 22:26:21]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/jdduke@chromium.org/484373004/1

[commit-bot: I haz the power, 2014-08-27 23:23:20]

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

[commit-bot: I haz the power, 2014-08-27 23:23:21]

Failed to commit the patch.

[aelias_OOO_until_Jul13, 2014-08-27 23:52:33]

The CQ bit was checked by aelias@chromium.org

[commit-bot: I haz the power, 2014-08-28 00:07:39]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/jdduke@chromium.org/484373004/1

[commit-bot: I haz the power, 2014-08-28 00:30:45]

Committed patchset #1 (id:1) as 37b60fb9573707e4510ac57cf145d7f7709533ac

[commit-bot: I haz the power, 2014-09-10 02:56:03]

Patchset 1 (id:??) landed as
https://crrev.com/46c0ec38bed034b6fcaade3f23258bd796a93db6
Cr-Commit-Position: refs/heads/master@{#292280}