Remove copyToRenderTarget methods. These Renderer methods can be replaced with virtual TextureStorage methods, now that we do not have TextureStorageInterface to complicate matters. This reduces the number of stubs in Renderer as well as allowing cleaner code re-use. BUG=angle:729 Change-Id: I6d2004d4f1abdb1041420144cee1c173e5ab199e Reviewed-on: https://chromium-review.googlesource.com/219839 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>