cc: Don't discard framebuffer when using partial swaps

cc/output/software_renderer.cc

Index: cc/output/software_renderer.cc
diff --git a/cc/output/software_renderer.cc b/cc/output/software_renderer.cc
index 394d79a7a35b290cea4ea8ce789837223bbd7833..fa8c178df35f8f425ed1735d844f9a911f4ce771 100644
--- a/cc/output/software_renderer.cc
+++ b/cc/output/software_renderer.cc
@@ -182,7 +182,11 @@ void SoftwareRenderer::ClearCanvas(SkColor color) {
     current_canvas_->clear(color);
 }
 
-void SoftwareRenderer::ClearFramebuffer(DrawingFrame* frame) {
+void SoftwareRenderer::DiscardPixels(bool has_external_stencil_test,
+                                     bool draw_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 {