Coverity: Fix pass by value errors in HostZoomMap::SetZoomLevel() function.

content/browser/host_zoom_map_impl.cc

 // Copyright (c) 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.
 
 #include <cmath>
 
 #include "content/browser/host_zoom_map_impl.h"
 
 #include "base/string_piece.h"
 #include "base/utf_string_conversions.h"
@@ skipping 53 matching lines @@
     host_zoom_levels_[i->first] = i->second;
   }
 }
 
 double HostZoomMapImpl::GetZoomLevel(const std::string& host) const {
   base::AutoLock auto_lock(lock_);
   HostZoomLevels::const_iterator i(host_zoom_levels_.find(host));
   return (i == host_zoom_levels_.end()) ? default_zoom_level_ : i->second;
 }
 
-void HostZoomMapImpl::SetZoomLevel(std::string host, double level) {
+void HostZoomMapImpl::SetZoomLevel(const std::string& host, double level) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
 
   {
     base::AutoLock auto_lock(lock_);
 
     if (content::ZoomValuesEqual(level, default_zoom_level_))
       host_zoom_levels_.erase(host);
     else
       host_zoom_levels_[host] = level;
   }
@@ skipping 92 matching lines @@
       }
       break;
     }
     default:
       NOTREACHED() << "Unexpected preference observed.";
   }
 }
 
 HostZoomMapImpl::~HostZoomMapImpl() {
 }