Fix new -Wnull-conversion violations detected by a newer clang.

chrome/browser/extensions/api/offscreen_tabs/offscreen_tabs_api.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/extensions/api/offscreen_tabs/offscreen_tabs_api.h"
 
 #include <algorithm>
 #include <vector>
 
 #include "base/hash_tables.h"
@@ skipping 526 matching lines @@
   result_.reset(offscreen_tab->CreateValue());
   return true;
 }
 
 GetAllOffscreenTabFunction::GetAllOffscreenTabFunction() {}
 GetAllOffscreenTabFunction::~GetAllOffscreenTabFunction() {}
 
 bool GetAllOffscreenTabFunction::RunImpl() {
   WebContents* web_contents = GetCurrentWebContents(this, &error_);
   if (!web_contents)
-    return NULL;
+    return false;
 
   ParentTab* parent_tab = GetMap()->GetParentTab(web_contents, extension_id());
   ListValue* tab_list = new ListValue();
   if (parent_tab) {
     for (OffscreenTabs::const_iterator i = parent_tab->offscreen_tabs().begin();
          i != parent_tab->offscreen_tabs().end(); ++i)
       tab_list->Append((*i)->CreateValue());
   }
 
   result_.reset(tab_list);
@@ skipping 273 matching lines @@
   // async case (when a "javascript": URL is sent to a tab).
   if (!is_async)
     SendResponse(true);
 
   return true;
 }
 
 void UpdateOffscreenTabFunction::PopulateResult() {
   // There's no result associated with this callback.
 }