Bug 1271002. Notify right away after getting the first frame of an animated image. r=seth

The decoding loop in Decoder::Decode only pauses to report progress when it runs out of bytes to decode. So for long animated images where the network is keeping up with decoding it will be a relatively long time until we deliver the first frame complete notification and corresponding invalidation. In most cases this shouldn't be too expensive as it is just dispatching a runnable to the main thread from the decoding thread.

image/DecodePool.cpp

@@ -469,6 +469,7 @@ void
 DecodePool::NotifyProgress(Decoder* aDecoder)
 {
   MOZ_ASSERT(aDecoder);
+  MOZ_ASSERT(aDecoder->HasProgress() && !aDecoder->IsMetadataDecode());
 
   if (!NS_IsMainThread() ||
       (aDecoder->GetDecoderFlags() & DecoderFlags::ASYNC_NOTIFY)) {

image/DecodePool.h

@@ -79,6 +79,11 @@ public:
    */
   already_AddRefed<nsIEventTarget> GetIOEventTarget();
 
+  /**
+   * Notify about progress on aDecoder.
+   */
+  void NotifyProgress(Decoder* aDecoder);
+
 private:
   friend class DecodePoolWorker;
 
@@ -87,7 +92,6 @@ private:
 
   void Decode(Decoder* aDecoder);
   void NotifyDecodeComplete(Decoder* aDecoder);
-  void NotifyProgress(Decoder* aDecoder);
 
   static StaticRefPtr<DecodePool> sSingleton;
   static uint32_t sNumCores;

image/Decoder.cpp

@@ -449,6 +449,12 @@ Decoder::PostFrameStop(Opacity aFrameOpacity
     mInvalidRect.UnionRect(mInvalidRect,
                            gfx::IntRect(gfx::IntPoint(0, 0), GetSize()));
   }
+
+  // If we are going to keep decoding we should notify now about the first frame being done.
+  if (mImage && mFrameCount == 1 && HasAnimation()) {
+    MOZ_ASSERT(HasProgress());
+    DecodePool::Singleton()->NotifyProgress(this);
+  }
 }
 
 void