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

content/public/browser/host_zoom_map.h

 // 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.
 
 #ifndef CONTENT_PUBLIC_BROWSER_HOST_ZOOM_MAP_H_
 #define CONTENT_PUBLIC_BROWSER_HOST_ZOOM_MAP_H_
 #pragma once
 
 #include <map>
 #include <string>
@@ skipping 27 matching lines @@
   // negative (to zoom out).
   //
   // This may be called on any thread.
   virtual double GetZoomLevel(const std::string& host) const = 0;
 
   // Sets the zoom level for the host or spec for a given url to |level|.  If
   // the level matches the current default zoom level, the host is erased
   // from the saved preferences; otherwise the new value is written out.
   //
   // This should only be called on the UI thread.
-  virtual void SetZoomLevel(std::string host, double level) = 0;
+  virtual void SetZoomLevel(const std::string& host, double level) = 0;
 
   // Get/Set the default zoom level for pages that don't override it.
   virtual double GetDefaultZoomLevel() const = 0;
   virtual void SetDefaultZoomLevel(double level) = 0;;
 
  protected:
   virtual ~HostZoomMap() {}
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_HOST_ZOOM_MAP_H_