[url] Reserve size in a smarter way to account for pre-allocated buffers

url/url_canon_stdstring.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 URL_URL_CANON_STDSTRING_H_
 #define URL_URL_CANON_STDSTRING_H_
 
 // This header file defines a canonicalizer output method class for STL
 // strings. Because the canonicalizer tries not to be dependent on the STL,
 // we have segregated it here.
 
 #include <string>
 
 #include "base/compiler_specific.h"
 #include "base/strings/string_piece.h"
 #include "url/url_canon.h"
 #include "url/url_export.h"
 
 namespace url {
 
 // Write into a std::string given in the constructor. This object does not own
 // the string itself, and the user must ensure that the string stays alive
 // throughout the lifetime of this object.
 //
 // The given string will be appended to; any existing data in the string will
-// be preserved. The caller should reserve() the amount of data in the string
-// they expect to be written. We will resize if necessary, but that's slow.
+// be preserved.
 //
 // Note that when canonicalization is complete, the string will likely have
 // unused space at the end because we make the string very big to start out
 // with (by |initial_size|). This ends up being important because resize
 // operations are slow, and because the base class needs to write directly
 // into the buffer.
 //
 // Therefore, the user should call Complete() before using the string that
 // this class wrote into.
 class URL_EXPORT StdStringCanonOutput : public CanonOutput {
@@ skipping 40 matching lines @@
     this->SetQuery(s.data(), Component(0, static_cast<int>(s.length())));
   }
   void SetRefStr(const base::BasicStringPiece<STR>& s) {
     this->SetRef(s.data(), Component(0, static_cast<int>(s.length())));
   }
 };
 
 }  // namespace url
 
 #endif  // URL_URL_CANON_STDSTRING_H_