Implemented Missing Virtual Functions Declaration in DirectRenderer Class.

cc/output/direct_renderer.h

Index: cc/output/direct_renderer.h
diff --git a/cc/output/direct_renderer.h b/cc/output/direct_renderer.h
index dcff365726639407273146a087fea82699869e79..c93e2b974bd754cfb176b01a45e4b1f1d0270d63 100644
--- a/cc/output/direct_renderer.h
+++ b/cc/output/direct_renderer.h
@@ -63,6 +63,7 @@ class CC_EXPORT DirectRenderer {
   virtual void SwapBuffersComplete() {}
   virtual void DidReceiveTextureInUseResponses(
       const gpu::TextureInUseResponses& responses) {}
+  virtual void ReceiveSwapBuffersAck(const CompositorFrameAck& ack) {}
 
   // Allow tests to enlarge the texture size of non-root render passes to
   // verify cases where the texture doesn't match the render pass size.
@@ -168,6 +169,9 @@ class CC_EXPORT DirectRenderer {
   virtual void EnsureScissorTestEnabled() = 0;
   virtual void EnsureScissorTestDisabled() = 0;
   virtual void DidChangeVisibility() = 0;
+  virtual void DiscardBackbuffer() {}
+  virtual void EnsureBackbuffer() {}
+
   virtual void CopyCurrentRenderPassToBitmap(
       DrawingFrame* frame,
       std::unique_ptr<CopyOutputRequest> request) = 0;