Move GetAttachmentRenderTarget and GetAttachmentSerial to FramebufferD3D.h. BUG=angle:841 Change-Id: Ic341239e848f1daf7cf38f233d79a58f221780f4 Reviewed-on: https://chromium-review.googlesource.com/232969 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>