Add test for SpdyReadQueue::Clear().

net/spdy/spdy_read_queue.h

 // Copyright (c) 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 NET_SPDY_SPDY_BUFFER_QUEUE_H_
 #define NET_SPDY_SPDY_BUFFER_QUEUE_H_
 
 #include <cstddef>
 #include <deque>
 #include <memory>
@@ skipping 22 matching lines @@
   void Enqueue(std::unique_ptr<SpdyBuffer> buffer);
 
   // Dequeues up to |len| (which must be positive) bytes into
   // |out|. Returns the number of bytes dequeued.
   size_t Dequeue(char* out, size_t len);
 
   // Removes all bytes from the queue.
   void Clear();
 
  private:
+  // Class invariant:
+  // |total_size_| is the sum of GetRemainingSize() of |queue_|'s elements.
   std::deque<std::unique_ptr<SpdyBuffer>> queue_;
   size_t total_size_;
 
   DISALLOW_COPY_AND_ASSIGN(SpdyReadQueue);
 };
 
 }  // namespace net
 
 #endif  // NET_SPDY_SPDY_BUFFER_QUEUE_H_