cc: Making BeginFrameSources support multiple observers.

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 ce6ccc4789107ff75f2d888b0ab167d47eba5923..48893f9bb1d8ad5191bde07c342f3092282ed9fe 100644
--- a/cc/test/scheduler_test_common.h
+++ b/cc/test/scheduler_test_common.h
@@ -92,11 +92,21 @@ class FakeBeginFrameSource : public BeginFrameSourceBase {
   FakeBeginFrameSource() : remaining_frames_(false) {}
   ~FakeBeginFrameSource() override {}
 
-  BeginFrameObserver* GetObserver() { return observer_; }
+  BeginFrameObserver* GetObserver() {
+    if ((observer_list_).might_have_observers()) {
+      base::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();
   }