Fix issue with webview background painting.
This CL fixes an issue with the background not being painted when hiding
the bookmark bar on the NTP. This was introduced by r304747 because the
WebContents does not have fast resize set at the time of the Paint() call,
causing the background to not be painted.
This CL restores the original behavior and allows clients to set the
color to paint into the background, which the app list sets as
transparent.
BUG=425444,435035
Review URL: https://codereview.chromium.org/
756513003
Cr-Commit-Position: refs/heads/master@{#305556}