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

cc/test/render_pass_test_utils.cc

Index: cc/test/render_pass_test_utils.cc
diff --git a/cc/test/render_pass_test_utils.cc b/cc/test/render_pass_test_utils.cc
index 1f2131635f4276a59332fc285dcc2df867b07592..049cf52ef1bde737ff9ac4267af066c502096860 100644
--- a/cc/test/render_pass_test_utils.cc
+++ b/cc/test/render_pass_test_utils.cc
@@ -110,7 +110,8 @@ void AddRenderPassQuad(TestRenderPass* to_pass,
                        TestRenderPass* contributing_pass,
                        ResourceProvider::ResourceId mask_resource_id,
                        const FilterOperations& filters,
-                       gfx::Transform transform) {
+                       gfx::Transform transform,
+                       SkXfermode::Mode blend_mode) {
   gfx::Rect output_rect = contributing_pass->output_rect;
   SharedQuadState* shared_state = to_pass->CreateAndAppendSharedQuadState();
   shared_state->SetAll(transform,
@@ -119,7 +120,7 @@ void AddRenderPassQuad(TestRenderPass* to_pass,
                        output_rect,
                        false,
                        1,
-                       SkXfermode::kSrcOver_Mode,
+                       blend_mode,
                        0);
   RenderPassDrawQuad* quad =
       to_pass->CreateAndAppendDrawQuad<RenderPassDrawQuad>();