[vr] Add a frame rate counter for the vr UI

cc/trees/frame_rate_counter.h

 // Copyright 2012 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_TREES_FRAME_RATE_COUNTER_H_
 #define CC_TREES_FRAME_RATE_COUNTER_H_
 
 #include <stddef.h>
 
 #include <memory>
 
 #include "base/macros.h"
 #include "base/time/time.h"
 #include "cc/base/ring_buffer.h"
+#include "cc/cc_export.h"
 
 namespace cc {
 
 // This class maintains a history of timestamps, and provides functionality to
 // intelligently compute average frames per second.
-class FrameRateCounter {
+class CC_EXPORT FrameRateCounter {
  public:
   static std::unique_ptr<FrameRateCounter> Create(bool has_impl_thread);
 
   size_t current_frame_number() const { return ring_buffer_.CurrentIndex(); }
   int dropped_frame_count() const { return dropped_frame_count_; }
   size_t time_stamp_history_size() const { return ring_buffer_.BufferSize(); }
 
   void SaveTimeStamp(base::TimeTicks timestamp, bool software);
 
   // n = 0 returns the oldest frame interval retained in the history, while n =
@@ skipping 20 matching lines @@
 
   bool has_impl_thread_;
   int dropped_frame_count_;
 
   DISALLOW_COPY_AND_ASSIGN(FrameRateCounter);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_FRAME_RATE_COUNTER_H_