Eliminate typed render target Texture attachment method. More convergance to the base TextureAttachment class. BUG=angle:732 Change-Id: I9aae1a49ab3908a4eb4cec4bce201b88a7d93926 Reviewed-on: https://chromium-review.googlesource.com/213859 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Shannon Woods <shannonwoods@chromium.org>