[cc] Show GPU memory usage and overage below FPS counter

cc/ring_buffer.h

 // Copyright 2013 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_RING_BUFFER_H_
 #define CC_RING_BUFFER_H_
 
 #include "base/logging.h"
 
 namespace cc {
@@ skipping 13 matching lines @@
     return current_index_;
   }
 
   // tests if a value was saved to this index
   bool IsFilledIndex(size_t n) const {
     return BufferIndex(n) < current_index_;
   }
 
   // n = 0 returns the oldest value and
   // n = bufferSize() - 1 returns the most recent value.
-  T ReadBuffer(size_t n) const {
+  const T& ReadBuffer(size_t n) const {
     DCHECK(IsFilledIndex(n));
     return buffer_[BufferIndex(n)];
   }
 
   void SaveToBuffer(T value) {
     buffer_[BufferIndex(0)] = value;
     current_index_++;
   }
 
  private:
   inline size_t BufferIndex(size_t n) const {
     return (current_index_ + n) % size;
   }
 
   T buffer_[size];
   size_t current_index_;
 };
 
 }  // namespace cc
 
 #endif  // CC_RING_BUFFER_H_