Expose a way to set a view's base background color.

BUG=263263

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=154891

Review URL: https://chromiumcodereview.appspot.com/19883002

git-svn-id: svn://svn.chromium.org/blink/trunk@154942 bbb929c8-8fbe-4397-9dbb-9b2b20218538