WebAudio: AudioScheduledSourceNode: Fix HasPendingActivity

HasPendingActivity() is called during garbage collection which may
happen during constructing an AudioNode. Since the handler has not been
set at this point we need to guard against derefencing null.

Bug: 937208
Change-Id: I4ee92d08c97f044aca8043443f04d751428e8455
Reviewed-on: https://chromium-review.googlesource.com/c/1495977Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636776}

third_party/blink/renderer/modules/webaudio/audio_node.cc

@@ -651,6 +651,10 @@ void AudioNode::SetHandler(scoped_refptr<AudioHandler> handler) {
 #endif
 }
 
+bool AudioNode::ContainsHandler() const {
+  return handler_.get();
+}
+
 AudioHandler& AudioNode::Handler() const {
   return *handler_;
 }

third_party/blink/renderer/modules/webaudio/audio_node.h

@@ -366,6 +366,11 @@ class MODULES_EXPORT AudioNode : public EventTargetWithInlineData {
   // This should be called in a constructor.
   void SetHandler(scoped_refptr<AudioHandler>);
 
+  // During construction time the handler may not be set properly. Since the
+  // garbage collector can call into HasPendingActivity() such calls need to be
+  // able to see whether a handle has been set.
+  bool ContainsHandler() const;
+
  private:
   void WarnIfContextClosed() const;
   void Dispose();

third_party/blink/renderer/modules/webaudio/audio_scheduled_source_node.cc

@@ -314,7 +314,8 @@ bool AudioScheduledSourceNode::HasPendingActivity() const {
 
   // If a node is scheduled or playing, do not collect the node prematurely
   // even its reference is out of scope. Then fire onended event if assigned.
-  return GetAudioScheduledSourceHandler().IsPlayingOrScheduled();
+  return ContainsHandler() &&
+         GetAudioScheduledSourceHandler().IsPlayingOrScheduled();
 }
 
 }  // namespace blink