[cc] Add SkBlendMode::kDstIn support to cc::Layer

cc/output/shader.h

Index: cc/output/shader.h
diff --git a/cc/output/shader.h b/cc/output/shader.h
index 6be4c5d83848ff7429974bc54df8a1b405146dae..c1e4d242d12d485bf7b6e5c1b03d78a246a4e041 100644
--- a/cc/output/shader.h
+++ b/cc/output/shader.h
@@ -87,6 +87,7 @@ enum SamplerType {
 enum BlendMode {
   BLEND_MODE_NONE,
   BLEND_MODE_NORMAL,
+  BLEND_MODE_DESTINATION_IN,
   BLEND_MODE_SCREEN,
   BLEND_MODE_OVERLAY,
   BLEND_MODE_DARKEN,
@@ -308,6 +309,7 @@ class FragmentShader {
 
   std::string GetHelperFunctions() const;
   std::string GetBlendFunction() const;
+  std::string GetBlendFunctionBodyForAlpha() const;
   std::string GetBlendFunctionBodyForRGB() const;
 
   DISALLOW_COPY_AND_ASSIGN(FragmentShader);