Set hbrBackground on wndclass, fixes white flash on resize

Set hbrBackground on wndclass, fixes white flash on resize

When in DWM, the DefWindowProc handling of WM_NCPAINT was clearing (to
white) in the area that was not yet drawn. When the wndclass's
background is set to COLOR_WINDOW this is avoided and the frame
appears to stay more "solid".

This only affects the white flash in the NC area in the linked bug,
not the grey in the content area. We should also consider reverting
the compositor.cc change from here http://crrev.com/192657 as it seems
to make resize a bit laggier.

R=sky@chromium.org
BUG=177115
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=221608

[sky, 2013-09-05 20:31:08]

LGTM

[commit-bot: I haz the power, 2013-09-05 20:50:40]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/scottmg@chromium.org/23596011/1

[commit-bot: I haz the power, 2013-09-06 00:21:59]

Retried try job too often on win7_aura for step(s) interactive_ui_tests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=win7_aura&...

[commit-bot: I haz the power, 2013-09-06 00:23:54]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/scottmg@chromium.org/23596011/1

[commit-bot: I haz the power, 2013-09-06 06:35:03]

Change committed as 221608