cc: Don't discard framebuffer when using partial swaps

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 efa34cf9d4d80d3cdb3a4da43d0d4300050aca70..3b50404396478313d6a0f35e7e51eb7a7c7488a0 100644
--- a/cc/output/direct_renderer.h
+++ b/cc/output/direct_renderer.h
@@ -118,7 +118,10 @@ class CC_EXPORT DirectRenderer : public Renderer {
                                         gfx::Rect target_rect) = 0;
   virtual void SetDrawViewport(gfx::Rect window_space_viewport) = 0;
   virtual void SetScissorTestRect(gfx::Rect scissor_rect) = 0;
-  virtual void ClearFramebuffer(DrawingFrame* frame) = 0;
+  virtual void DiscardPixels(bool has_external_stencil_test,
+                             bool redraw_rect_covers_full_surface) = 0;
+  virtual void ClearFramebuffer(DrawingFrame* frame,
+                                bool has_external_stencil_test) = 0;
   virtual void DoDrawQuad(DrawingFrame* frame, const DrawQuad* quad) = 0;
   virtual void BeginDrawingFrame(DrawingFrame* frame) = 0;
   virtual void FinishDrawingFrame(DrawingFrame* frame) = 0;