Re-land "Android, video: clean-up WebMediaPlayerClientImpl::paintOnAndroid()."

Remove unnecessary OwnPtr<WebGraphicsContext3DProvider> object construction.

r169101 is reverted because of invalid ASSERT. When WebRTC is the source of a
video, WebMediaPlayer::LoadTypeMediaStream is possible.

BUG=349871

Review URL: https://codereview.chromium.org/199423002

git-svn-id: svn://svn.chromium.org/blink/trunk@169219 bbb929c8-8fbe-4397-9dbb-9b2b20218538