Implement mix-blend-mode=screen using glBlendFunc.

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 75a5d6e720e76572268f2a7b5e32570c24647d83..352766194430370b0a9ce9d93f5227cca38f7434 100644
--- a/cc/output/gl_renderer.h
+++ b/cc/output/gl_renderer.h
@@ -143,6 +143,11 @@ class CC_EXPORT GLRenderer : public DirectRenderer {
                             const CheckerboardDrawQuad* quad);
   void DrawDebugBorderQuad(const DrawingFrame* frame,
                            const DebugBorderDrawQuad* quad);
+  static bool ShouldApplyBackgroundFilters(DrawingFrame* frame,
+                                           const RenderPassDrawQuad* quad);
+  static bool ShouldApplyBlendModeUsingBlendFunc(const DrawQuad* quad);
+  void ApplyBlendModeUsingBlendFunc(const DrawQuad* quad);
+  void RestoreBlendFuncToDefault(const DrawQuad* quad);
   scoped_ptr<ScopedResource> GetBackgroundWithFilters(
       DrawingFrame* frame,
       const RenderPassDrawQuad* quad,