Clear root window when the aura root window is moved/resized

Without this, switching display may leave some content on other display.

BUG=152003
TEST=manual: Switch to external/internal monitor doesn't leave the content.
 Manually killed chrome (with /var/run/disable_chrome_restart) and confirmed that the content wasn't cleared.

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=158757

Review URL: https://codereview.chromium.org/10981024

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@158771 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed