Make sure WebContents passed to chrome_page_zoom::Zoom() have a ZoomController.

chrome/browser/chrome_page_zoom.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 "chrome/browser/chrome_page_zoom.h"
 
 #include <algorithm>
 #include <cmath>
 
 #include "base/prefs/pref_service.h"
@@ skipping 51 matching lines @@
   return PresetZoomValues(PAGE_ZOOM_VALUE_TYPE_FACTOR, custom_factor);
 }
 
 std::vector<double> PresetZoomLevels(double custom_level) {
   return PresetZoomValues(PAGE_ZOOM_VALUE_TYPE_LEVEL, custom_level);
 }
 
 void Zoom(content::WebContents* web_contents, content::PageZoom zoom) {
   ZoomController* zoom_controller =
       ZoomController::FromWebContents(web_contents);
+  DCHECK(zoom_controller);
 
   double current_zoom_level = zoom_controller->GetZoomLevel();
   double default_zoom_level =
       Profile::FromBrowserContext(web_contents->GetBrowserContext())->
           GetPrefs()->GetDouble(prefs::kDefaultZoomLevel);
 
   if (zoom == content::PAGE_ZOOM_RESET) {
     zoom_controller->SetZoomLevel(default_zoom_level);
     content::RecordAction(UserMetricsAction("ZoomNormal"));
     return;
@@ skipping 30 matching lines @@
         zoom_controller->SetZoomLevel(zoom_level);
         content::RecordAction(UserMetricsAction("ZoomPlus"));
         return;
       }
     }
     content::RecordAction(UserMetricsAction("ZoomPlus_AtMaximum"));
   }
 }
 
 }  // namespace chrome_page_zoom