Bug 1400407 - Part 3: Remove IsMirror concept, as it is no longer used by WebVR,r=jgilbert

- This patch is the same as one from Bug 1382104 (Remove IsMirror concept
  in favor of checking forwarder).
- It is safe to uplift this patch without the rest of Bug 1382104 as long
  as the remaining Bug 1381084 is also uplifted.

MozReview-Commit-ID: 21YZObeSUa3

gfx/layers/client/CanvasClient.cpp

@@ -410,11 +410,6 @@ CanvasClientSharedSurface::UpdateRenderer(gfx::IntSize aSize, Renderer& aRendere
       gfxCriticalError() << "Invalid canvas front buffer";
       return;
     }
-  } else if (canvasRenderer && canvasRenderer->mIsMirror) {
-    mShSurfClient = CloneSurface(gl->Screen()->Front()->Surf(), canvasRenderer->mFactory.get());
-    if (!mShSurfClient) {
-      return;
-    }
   } else {
     mShSurfClient = gl->Screen()->Front();
     if (mShSurfClient && mShSurfClient->GetAllocator() &&