Remove "mixin" from BeginFrameSource/Observer base classes.

cc/test/fake_external_begin_frame_source.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_TEST_FAKE_EXTERNAL_BEGIN_FRAME_SOURCE_H_
 #define CC_TEST_FAKE_EXTERNAL_BEGIN_FRAME_SOURCE_H_
 
 #include "base/memory/weak_ptr.h"
 #include "cc/scheduler/begin_frame_source.h"
 
 namespace cc {
 
 class FakeExternalBeginFrameSource
-    : public BeginFrameSourceMixIn,
+    : public BeginFrameSourceBase,
       public NON_EXPORTED_BASE(base::NonThreadSafe) {
  public:
   explicit FakeExternalBeginFrameSource(double refresh_rate);
   ~FakeExternalBeginFrameSource() override;
 
   bool is_ready() const { return is_ready_; }
 
   // BeginFrameSource implementation.
   void SetClientReady() override;
 
-  // BeginFrameSourceMixIn overrides.
+  // BeginFrameSourceBase overrides.
   void OnNeedsBeginFramesChange(bool needs_begin_frames) override;
 
   void TestOnBeginFrame();
   void PostTestOnBeginFrame();
 
  private:
   double milliseconds_per_frame_;
   bool is_ready_;
   base::WeakPtrFactory<FakeExternalBeginFrameSource> weak_ptr_factory_;
 };
 
 }  // namespace cc
 
 #endif  // CC_TEST_FAKE_EXTERNAL_BEGIN_FRAME_SOURCE_H_