Implement CHROMIUM_copy_compressed_texture for D3D11.

BUG=angleproject:1356

Change-Id: Id563997d2921cf558c52a781ae66d8bde58d1f2f
Reviewed-on: https://chromium-review.googlesource.com/339847
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>
diff --git a/src/libANGLE/Renderbuffer.cpp b/src/libANGLE/Renderbuffer.cpp
index 977117d..7b30a46 100644
--- a/src/libANGLE/Renderbuffer.cpp
+++ b/src/libANGLE/Renderbuffer.cpp
@@ -95,17 +95,12 @@
     return NoError();
 }
 
-rx::RenderbufferImpl *Renderbuffer::getImplementation()
+rx::RenderbufferImpl *Renderbuffer::getImplementation() const
 {
     ASSERT(mRenderbuffer);
     return mRenderbuffer;
 }
 
-const rx::RenderbufferImpl *Renderbuffer::getImplementation() const
-{
-    return mRenderbuffer;
-}
-
 GLsizei Renderbuffer::getWidth() const
 {
     return mWidth;