Remove "mixin" from BeginFrameSource/Observer base classes.

cc/scheduler/scheduler.h

Index: cc/scheduler/scheduler.h
diff --git a/cc/scheduler/scheduler.h b/cc/scheduler/scheduler.h
index 13daf008cc338935212784a6e4b188215ce54b2e..de182b42bb42997b93e74998b98d2d2949cc9e22 100644
--- a/cc/scheduler/scheduler.h
+++ b/cc/scheduler/scheduler.h
@@ -72,7 +72,7 @@ class CC_EXPORT SchedulerFrameSourcesConstructor {
   friend class Scheduler;
 };
 
-class CC_EXPORT Scheduler : public BeginFrameObserverMixIn {
+class CC_EXPORT Scheduler : public BeginFrameObserverBase {
  public:
   static scoped_ptr<Scheduler> Create(
       SchedulerClient* client,
@@ -91,7 +91,7 @@ class CC_EXPORT Scheduler : public BeginFrameObserverMixIn {
 
   ~Scheduler() override;
 
-  // BeginFrameObserverMixin
+  // BeginFrameObserverBase
   bool OnBeginFrameMixInDelegate(const BeginFrameArgs& args) override;
 
   void OnDrawForOutputSurface();