Cleanup: Pass std::string as const reference if possible

components/network_hints/renderer/dns_prefetch_queue.h

 // Copyright (c) 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.
 
 // DnsQueue is implemented as an almost FIFO circular buffer for text
 // strings that don't have embedded nulls ('\0').  The "almost" element is that
 // some duplicate strings may be removed (i.e., the string won't really be
 // pushed *if* the class happens to notice that a duplicate is already in the
 // queue).
 // The buffers internal format is null terminated character strings
@@ skipping 45 matching lines @@
   // Push takes an unterminated string of the given length
   // and inserts it into the queue for later
   // extraction by read.  For each successful push(), there
   // can later be a corresponding read() to extracted the text.
   // The string must not contain an embedded null terminator
   // Exactly length chars are written, or the push fails (where
   // "fails" means nothing is written).
   // Returns true for success, false for failure (nothing written).
   PushResult Push(const char* source, const size_t length);
 
-  PushResult Push(std::string source) {
+  PushResult Push(const std::string& source) {
     return Push(source.c_str(), source.length());
   }
 
   // Extract the next available string from the buffer.
   // If the buffer is empty, then return false.
   bool Pop(std::string* out_string);
 
  private:
   bool Validate();  // Checks that all internal data is valid.
 
   const scoped_ptr<char[]> buffer_;  // Circular buffer, plus extra char ('\0').
   const BufferSize buffer_size_;  // Size one smaller than allocated space.
   const BufferSize buffer_sentinel_;  // Index of extra '\0' at end of buffer_.
 
   // If writable_ == readable_, then the buffer is empty.
   BufferSize readable_;  // Next readable char in buffer_.
   BufferSize writeable_;  // The next space in buffer_ to push.
 
   // Number of queued strings
   size_t size_;
 
   DISALLOW_COPY_AND_ASSIGN(DnsQueue);
 };  // class DnsQueue
 
 }  // namespace network_hints
 
 #endif  // COMPONENTS_NETWORK_HINTS_RENDERER_DNS_PREFETCH_QUEUE_H__