Remove "mixin" from BeginFrameSource/Observer base classes.

cc/scheduler/scheduler_unittest.cc

 // Copyright 2011 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.
 
 #include "cc/scheduler/scheduler.h"
 
 #include <string>
 #include <vector>
 
 #include "base/logging.h"
@@ skipping 210 matching lines @@
   base::TimeDelta CommitToActivateDurationEstimate() override {
     return commit_to_activate_duration_;
   }
 
  private:
   base::TimeDelta draw_duration_;
   base::TimeDelta begin_main_frame_to_commit_duration_;
   base::TimeDelta commit_to_activate_duration_;
 };
 
-class FakeExternalBeginFrameSource : public BeginFrameSourceMixIn {
+class FakeExternalBeginFrameSource : public BeginFrameSourceBase {
  public:
   explicit FakeExternalBeginFrameSource(FakeSchedulerClient* client)
       : client_(client) {}
   ~FakeExternalBeginFrameSource() override {}
 
   void OnNeedsBeginFramesChange(bool needs_begin_frames) override {
     if (needs_begin_frames) {
       client_->PushAction("SetNeedsBeginFrames(true)");
     } else {
       client_->PushAction("SetNeedsBeginFrames(false)");
@@ skipping 2756 matching lines @@
   EXPECT_SCOPED(AdvanceFrame());
 
   // At the next BeginFrame, authoritative interval is used instead of previous
   // interval.
   EXPECT_NE(initial_interval, scheduler_->BeginImplFrameInterval());
   EXPECT_EQ(authoritative_interval, scheduler_->BeginImplFrameInterval());
 }
 
 }  // namespace
 }  // namespace cc