Bug 1736266 - Remove unused DidCompositeObserver. r=sotaro

This was added in bug 1320864 and looks like it was used for plugins.

Differential Revision: https://phabricator.services.mozilla.com/D128735

gfx/layers/wr/WebRenderLayerManager.cpp

@@ -602,11 +602,6 @@ void WebRenderLayerManager::DidComposite(
       mTransactionIdAllocator->NotifyTransactionCompleted(aTransactionId);
     }
   }
-
-  // These observers fire whether or not we were in a transaction.
-  for (size_t i = 0; i < mDidCompositeObservers.Length(); i++) {
-    mDidCompositeObservers[i]->DidComposite();
-  }
 }
 
 void WebRenderLayerManager::ClearCachedResources() {
@@ -673,18 +668,6 @@ TransactionId WebRenderLayerManager::GetLastTransactionId() {
   return mLatestTransactionId;
 }
 
-void WebRenderLayerManager::AddDidCompositeObserver(
-    DidCompositeObserver* aObserver) {
-  if (!mDidCompositeObservers.Contains(aObserver)) {
-    mDidCompositeObservers.AppendElement(aObserver);
-  }
-}
-
-void WebRenderLayerManager::RemoveDidCompositeObserver(
-    DidCompositeObserver* aObserver) {
-  mDidCompositeObservers.RemoveElement(aObserver);
-}
-
 void WebRenderLayerManager::FlushRendering(wr::RenderReasons aReasons) {
   CompositorBridgeChild* cBridge = GetCompositorBridgeChild();
   if (!cBridge) {

gfx/layers/wr/WebRenderLayerManager.h

@@ -54,10 +54,6 @@ class WebRenderBridgeChild;
 class WebRenderParentCommand;
 class TransactionIdAllocator;
 class LayerUserData;
-class DidCompositeObserver {
- public:
-  virtual void DidComposite() = 0;
-};
 
 class WebRenderLayerManager final : public WindowRenderer {
   typedef nsTArray<RefPtr<Layer>> LayerRefArray;
@@ -113,9 +109,6 @@ class WebRenderLayerManager final : public WindowRenderer {
   void SetTransactionIdAllocator(TransactionIdAllocator* aAllocator);
   TransactionId GetLastTransactionId();
 
-  void AddDidCompositeObserver(DidCompositeObserver* aObserver);
-  void RemoveDidCompositeObserver(DidCompositeObserver* aObserver);
-
   void FlushRendering(wr::RenderReasons aReasons) override;
   void WaitOnTransactionProcessed() override;
 
@@ -234,8 +227,6 @@ class WebRenderLayerManager final : public WindowRenderer {
   RefPtr<TransactionIdAllocator> mTransactionIdAllocator;
   TransactionId mLatestTransactionId;
 
-  nsTArray<DidCompositeObserver*> mDidCompositeObservers;
-
   gfx::UserData mUserData;
 
   // This holds the scroll data that we need to send to the compositor for