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

cc/layers/delegated_renderer_layer_impl_unittest.cc

Index: cc/layers/delegated_renderer_layer_impl_unittest.cc
diff --git a/cc/layers/delegated_renderer_layer_impl_unittest.cc b/cc/layers/delegated_renderer_layer_impl_unittest.cc
index 832a483bf06d938561353c0e357a5ee127924e0f..38253edd14c27d544053d5e515b2263f2a4e94cd 100644
--- a/cc/layers/delegated_renderer_layer_impl_unittest.cc
+++ b/cc/layers/delegated_renderer_layer_impl_unittest.cc
@@ -1414,7 +1414,12 @@ TEST_F(DelegatedRendererLayerImplTest, Occlusion) {
                                         pass1_id,
                                         gfx::Rect(layer_size),
                                         gfx::Transform());
-  AddRenderPassQuad(pass1, pass2, 0, FilterOperations(), transform);
+  AddRenderPassQuad(pass1,
+                    pass2,
+                    0,
+                    FilterOperations(),
+                    transform,
+                    SkXfermode::kSrcOver_Mode);
   delegated_renderer_layer_impl->SetFrameDataForRenderPasses(
       1.f, &delegated_render_passes);