Implement mix-blend-mode in GL renderer using shaders.

cc/output/program_binding.h

Index: cc/output/program_binding.h
diff --git a/cc/output/program_binding.h b/cc/output/program_binding.h
index 912329ef6bf548055ad6a07de577f12040af4936..95cd81a999e82741f7aba637f6e7c5af06b65580 100644
--- a/cc/output/program_binding.h
+++ b/cc/output/program_binding.h
@@ -58,13 +58,16 @@ class ProgramBinding : public ProgramBindingBase {
 
   void Initialize(ContextProvider* context_provider,
                   TexCoordPrecision precision,
-                  SamplerType sampler) {
+                  SamplerType sampler,
+                  SkXfermode::Mode blend_mode = SkXfermode::kSrcOver_Mode) {
     DCHECK(context_provider);
     DCHECK(!initialized_);
 
     if (context_provider->IsContextLost())
       return;
 
+    fragment_shader_.SetBlendMode(blend_mode);
+
     if (!ProgramBindingBase::Init(
             context_provider->ContextGL(),
             vertex_shader_.GetShaderString(),