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

Android doesn't support WebMediaPlayer::LoadTypeMediaStream so change "if
statement" to ASSERT. See WebMediaPlayerAndroid::load().

Remove unnecessary OwnPtr<WebGraphicsContext3DProvider> object construction.

BUG=349871

Review URL: https://codereview.chromium.org/196603006

git-svn-id: svn://svn.chromium.org/blink/trunk@169101 bbb929c8-8fbe-4397-9dbb-9b2b20218538