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

content/browser/host_zoom_map_impl.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_BROWSER_HOST_ZOOM_MAP_IMPL_H_
 #define CONTENT_BROWSER_HOST_ZOOM_MAP_IMPL_H_
 #pragma once
 
 #include <map>
 #include <string>
@@ skipping 13 matching lines @@
     : public NON_EXPORTED_BASE(content::HostZoomMap),
       public content::NotificationObserver,
       public base::SupportsUserData::Data {
  public:
   HostZoomMapImpl();
   virtual ~HostZoomMapImpl();
 
   // HostZoomMap implementation:
   virtual void CopyFrom(HostZoomMap* copy) OVERRIDE;
   virtual double GetZoomLevel(const std::string& host) const OVERRIDE;
-  virtual void SetZoomLevel(std::string host, double level) OVERRIDE;
+  virtual void SetZoomLevel(const std::string& host, double level) OVERRIDE;
   virtual double GetDefaultZoomLevel() const OVERRIDE;
   virtual void SetDefaultZoomLevel(double level) OVERRIDE;
 
   // Returns the temporary zoom level that's only valid for the lifetime of
   // the given WebContents (i.e. isn't saved and doesn't affect other
   // WebContentses) if it exists, the default zoom level otherwise.
   //
   // This may be called on any thread.
   double GetTemporaryZoomLevel(int render_process_id,
                                int render_view_id) const;
@@ skipping 31 matching lines @@
   // Used around accesses to |host_zoom_levels_|, |default_zoom_level_| and
   // |temporary_zoom_levels_| to guarantee thread safety.
   mutable base::Lock lock_;
 
   content::NotificationRegistrar registrar_;
 
   DISALLOW_COPY_AND_ASSIGN(HostZoomMapImpl);
 };
 
 #endif  // CONTENT_BROWSER_HOST_ZOOM_MAP_IMPL_H_