cc: Don't discard framebuffer when using partial swaps

cc/output/software_renderer.cc

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/output/software_renderer.h"
 
 #include "base/debug/trace_event.h"
 #include "cc/base/math_util.h"
 #include "cc/output/compositor_frame.h"
 #include "cc/output/compositor_frame_ack.h"
@@ skipping 164 matching lines @@
 
 void SoftwareRenderer::ClearCanvas(SkColor color) {
   // SkCanvas::clear doesn't respect the current clipping region
   // so we SkCanvas::drawColor instead if scissoring is active.
   if (is_scissor_enabled_)
     current_canvas_->drawColor(color, SkXfermode::kSrc_Mode);
   else
     current_canvas_->clear(color);
 }
 
-void SoftwareRenderer::ClearFramebuffer(DrawingFrame* frame) {
+void SoftwareRenderer::DiscardPixels(bool has_external_stencil_test,
+                                     bool redraw_rect_covers_full_surface) {
+}
+
+void SoftwareRenderer::ClearFramebuffer(DrawingFrame* frame,
+                                        bool has_external_stencil_test) {
   if (frame->current_render_pass->has_transparent_background) {
     ClearCanvas(SkColorSetARGB(0, 0, 0, 0));
   } else {
 #ifndef NDEBUG
     // On DEBUG builds, opaque render passes are cleared to blue
     // to easily see regions that were not drawn on the screen.
     ClearCanvas(SkColorSetARGB(255, 0, 0, 255));
 #endif
   }
 }
@@ skipping 349 matching lines @@
     EnsureBackbuffer();
   else
     DiscardBackbuffer();
 }
 
 void SoftwareRenderer::SetDiscardBackBufferWhenNotVisible(bool discard) {
   // The software renderer always discards the backbuffer when not visible.
 }
 
 }  // namespace cc