Use GetAs/GetImplAs whenever possible.

This patch cleans up the rest of our custom casting helper
functions.

Change-Id: I41975c736765fca855c4498acca31116df3e8317
Reviewed-on: https://chromium-review.googlesource.com/263477
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
diff --git a/src/libANGLE/renderer/d3d/FramebufferD3D.cpp b/src/libANGLE/renderer/d3d/FramebufferD3D.cpp
index 1d994ff..8e6aa90 100644
--- a/src/libANGLE/renderer/d3d/FramebufferD3D.cpp
+++ b/src/libANGLE/renderer/d3d/FramebufferD3D.cpp
@@ -387,7 +387,7 @@
     {
         gl::Renderbuffer *renderbuffer = attachment->getRenderbuffer();
         ASSERT(renderbuffer);
-        RenderbufferD3D *renderbufferD3D = RenderbufferD3D::makeRenderbufferD3D(renderbuffer->getImplementation());
+        RenderbufferD3D *renderbufferD3D = GetImplAs<RenderbufferD3D>(renderbuffer);
         *outRT = renderbufferD3D->getRenderTarget();
         return gl::Error(GL_NO_ERROR);
     }
@@ -431,7 +431,7 @@
     {
         gl::Renderbuffer *renderbuffer = attachment->getRenderbuffer();
         ASSERT(renderbuffer);
-        RenderbufferD3D *renderbufferD3D = RenderbufferD3D::makeRenderbufferD3D(renderbuffer->getImplementation());
+        RenderbufferD3D *renderbufferD3D = GetImplAs<RenderbufferD3D>(renderbuffer);
         return renderbufferD3D->getRenderTargetSerial();
     }
     else if (attachment->type() == GL_FRAMEBUFFER_DEFAULT)