cc: Making BeginFrameSources support multiple BeginFrameObservers.

cc/test/scheduler_test_common.h

Index: cc/test/scheduler_test_common.h
diff --git a/cc/test/scheduler_test_common.h b/cc/test/scheduler_test_common.h
index 763ccc8351d761e5d546b961cbfa29d17f424e37..897e0bab7ab3c306a7496ff0e5ec3ff44dfce599 100644
--- a/cc/test/scheduler_test_common.h
+++ b/cc/test/scheduler_test_common.h
@@ -76,11 +76,21 @@ class TestDelayBasedTimeSource : public DelayBasedTimeSource {
 struct FakeBeginFrameSource : public BeginFrameSourceMixIn {
   bool remaining_frames_ = false;
 
-  BeginFrameObserver* GetObserver() { return observer_; }
+  BeginFrameObserver* GetObserver() {
+    if ((observer_list_).might_have_observers()) {
+      ObserverList<BeginFrameObserver>::Iterator it(&observer_list_);
+      BeginFrameObserver* obs;
+      while ((obs = it.GetNext()) != nullptr) {
+        return obs;
+      }
+    }
+    return nullptr;
+  }
 
   BeginFrameArgs TestLastUsedBeginFrameArgs() {
-    if (observer_) {
-      return observer_->LastUsedBeginFrameArgs();
+    BeginFrameObserver* obs = GetObserver();
+    if (obs) {
+      return obs->LastUsedBeginFrameArgs();
     }
     return BeginFrameArgs();
   }
@@ -90,7 +100,7 @@ struct FakeBeginFrameSource : public BeginFrameSourceMixIn {
 
   // BeginFrameSource
   void DidFinishFrame(size_t remaining_frames) override;
-  void AsValueInto(base::trace_event::TracedValue* dict) const override;
+  void AsValueInto(base::trace_event::TracedValue* dict) override;
 
   ~FakeBeginFrameSource() override {}
 };