[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
TBR=aelias@chromium.org

Review URL: https://codereview.chromium.org/484373004

Cr-Commit-Position: refs/heads/master@{#292280}
(cherry picked from commit 46c0ec38bed034b6fcaade3f23258bd796a93db6)

[jdduke (slow), 2014-09-02 15:31:03]

Committed patchset #1 (id:1) to pending queue manually as 33de363 (presubmit
successful).