Replace LayerImpl::sorting_context_id with transform reference.

cc/layers/solid_color_layer_impl_unittest.cc

Index: cc/layers/solid_color_layer_impl_unittest.cc
diff --git a/cc/layers/solid_color_layer_impl_unittest.cc b/cc/layers/solid_color_layer_impl_unittest.cc
index d2e2bb86dac9c30a47fa5bccd4fec018da37f473..d35b779d979fd14cb6ecbcbe7df3c25b57796e9b 100644
--- a/cc/layers/solid_color_layer_impl_unittest.cc
+++ b/cc/layers/solid_color_layer_impl_unittest.cc
@@ -149,10 +149,10 @@ TEST(SolidColorLayerImplTest, VerifyOpaqueRect) {
   layer->SetBackgroundColor(SkColorSetARGBInline(255, 10, 20, 30));
   EXPECT_TRUE(layer->contents_opaque());
   {
-    std::unique_ptr<SolidColorLayerImpl> layer_impl =
-        SolidColorLayerImpl::Create(host->host_impl()->active_tree(),
-                                    layer->id());
-    layer->PushPropertiesTo(layer_impl.get());
+    DebugScopedSetImplThread scoped_impl_thread(host->GetTaskRunnerProvider());
+    host->FinishCommitOnImplThread(host->host_impl());
+    LayerImpl* layer_impl =
+        host->host_impl()->active_tree()->LayerById(layer->id());
 
     // The impl layer should call itself opaque as well.
     EXPECT_TRUE(layer_impl->contents_opaque());
@@ -175,12 +175,12 @@ TEST(SolidColorLayerImplTest, VerifyOpaqueRect) {
   layer->SetBackgroundColor(SkColorSetARGBInline(254, 10, 20, 30));
   EXPECT_FALSE(layer->contents_opaque());
   {
-    std::unique_ptr<SolidColorLayerImpl> layer_impl =
-        SolidColorLayerImpl::Create(host->host_impl()->active_tree(),
-                                    layer->id());
-    layer->PushPropertiesTo(layer_impl.get());
+    DebugScopedSetImplThread scoped_impl_thread(host->GetTaskRunnerProvider());
+    host->FinishCommitOnImplThread(host->host_impl());
+    LayerImpl* layer_impl =
+        host->host_impl()->active_tree()->LayerById(layer->id());
 
-    // The impl layer should callnot itself opaque anymore.
+    // The impl layer should not call itself opaque anymore.
     EXPECT_FALSE(layer_impl->contents_opaque());
 
     // Impl layer has 1 opacity, but the color is not opaque, so the opaque_rect