Remove "mixin" from BeginFrameSource/Observer base classes.

cc/scheduler/begin_frame_source.h

Index: cc/scheduler/begin_frame_source.h
diff --git a/cc/scheduler/begin_frame_source.h b/cc/scheduler/begin_frame_source.h
index d845dacd8e417505e27b3663246ae38c53234285..4b5ee8a79f984210bde510388838d106b9370e79 100644
--- a/cc/scheduler/begin_frame_source.h
+++ b/cc/scheduler/begin_frame_source.h
@@ -60,11 +60,11 @@ class CC_EXPORT BeginFrameObserver {
 // Users of this mix in should;
 //  - Implement the OnBeginFrameMixInDelegate function.
 //  - Recommended (but not required) to call
-//    BeginFrameObserverMixIn::OnValueInto in their overridden OnValueInto
+//    BeginFrameObserverBase::OnValueInto in their overridden OnValueInto
 //    function.
-class CC_EXPORT BeginFrameObserverMixIn : public BeginFrameObserver {
+class CC_EXPORT BeginFrameObserverBase : public BeginFrameObserver {
  public:
-  BeginFrameObserverMixIn();
+  BeginFrameObserverBase();
 
   // BeginFrameObserver