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) {