[Merge to M52]Fill the canvas with black background before painting wallpaper.
[Merge to M52]Fill the canvas with black background before painting wallpaper.

Merge this CL to M52 on behalf of xiaoyinh@. This change is to fill the canvas
with black background every time before painting a new wallpaper, so that the
new wallpaper with partial transparency won't be painted upon a previous wallpaper.

BUG=606434
TBR=achuith@chromium.org, xiaoyinh@chromium.org

Review-Url: https://codereview.chromium.org/1996793002
Cr-Commit-Position: refs/heads/master@{#395175}
(cherry picked from commit 5fb57605dc341abde4f2eace5ec578520b74b224)

Review URL: https://codereview.chromium.org/2039173003 .

Cr-Commit-Position: 03d981dee550804c6934fd1a1d9cfa3c172affd3@{#244}
Cr-Branched-From: e9621a186760eb36d74098d9e33e4dd91c427b34@{#394939}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk