cc: Don't discard framebuffer when using partial swaps

cc/output/gl_renderer.h

Index: cc/output/gl_renderer.h
diff --git a/cc/output/gl_renderer.h b/cc/output/gl_renderer.h
index 13f65f2cc5c55395c557fbb051284ffe3c5e1f5d..70274b4984e1e456f1faabe81cf412056eb2fa46 100644
--- a/cc/output/gl_renderer.h
+++ b/cc/output/gl_renderer.h
@@ -117,7 +117,10 @@ class CC_EXPORT GLRenderer : public DirectRenderer {
                                         gfx::Rect target_rect) OVERRIDE;
   virtual void SetDrawViewport(gfx::Rect window_space_viewport) OVERRIDE;
   virtual void SetScissorTestRect(gfx::Rect scissor_rect) OVERRIDE;
-  virtual void ClearFramebuffer(DrawingFrame* frame) OVERRIDE;
+  virtual void DiscardPixels(bool has_external_stencil_test,
+                             bool redraw_rect_covers_full_surface) OVERRIDE;
+  virtual void ClearFramebuffer(DrawingFrame* frame,
+                                bool has_external_stencil_test) OVERRIDE;
   virtual void DoDrawQuad(DrawingFrame* frame, const class DrawQuad*) OVERRIDE;
   virtual void BeginDrawingFrame(DrawingFrame* frame) OVERRIDE;
   virtual void FinishDrawingFrame(DrawingFrame* frame) OVERRIDE;