linux: implement WebPluginDelegateProxy::BackgroundChanged
linux: implement WebPluginDelegateProxy::BackgroundChanged

That makes windowless transparent flash much better.

BUG=19157

Review URL: http://codereview.chromium.org/165522


git-svn-id: http://src.chromium.org/svn/trunk/src@23461 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: ec3432605a7b702602461fc6d12c054ce6bc83a2
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser