Bug 1217662 - part 3 - move nsDisplayBlendContainer::GetLayerState out-of-line; r=mattwoodrow

This method is virtual, so defining it inline isn't worth a whole lot.
Defining it inline also means that we require the complete definition of
LayerManager from Layers.h, and we're trying to avoid including Layers.h
whenever possible.  Let's move it out-of-line to solve this problem.

layout/base/nsDisplayList.cpp

@@ -4165,6 +4165,17 @@ nsDisplayBlendContainer::BuildLayer(nsDisplayListBuilder* aBuilder,
   return container.forget();
 }
 
+LayerState
+nsDisplayBlendContainer::GetLayerState(nsDisplayListBuilder* aBuilder,
+                                       LayerManager* aManager,
+                                       const ContainerLayerParameters& aParameters)
+{
+  if (mCanBeActive && aManager->SupportsMixBlendModes(mContainedBlendModes)) {
+    return mozilla::LAYER_ACTIVE;
+  }
+  return mozilla::LAYER_INACTIVE;
+}
+
 bool nsDisplayBlendContainer::TryMerge(nsDisplayListBuilder* aBuilder, nsDisplayItem* aItem) {
   if (aItem->GetType() != TYPE_BLEND_CONTAINER)
     return false;

layout/base/nsDisplayList.h

@@ -3286,13 +3286,7 @@ public:
                                                const ContainerLayerParameters& aContainerParameters) override;
     virtual LayerState GetLayerState(nsDisplayListBuilder* aBuilder,
                                      LayerManager* aManager,
-                                     const ContainerLayerParameters& aParameters) override
-    {
-      if (mCanBeActive && aManager->SupportsMixBlendModes(mContainedBlendModes)) {
-        return mozilla::LAYER_ACTIVE;
-      }
-      return mozilla::LAYER_INACTIVE;
-    }
+                                     const ContainerLayerParameters& aParameters) override;
     virtual bool TryMerge(nsDisplayListBuilder* aBuilder, nsDisplayItem* aItem) override;
     virtual bool ShouldFlattenAway(nsDisplayListBuilder* aBuilder) override {
       return false;