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

chrome/browser/debugger/devtools_window.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 <algorithm>
 
 #include "base/command_line.h"
 #include "base/json/json_writer.h"
 #include "base/lazy_instance.h"
 #include "base/string_number_conversions.h"
@@ skipping 83 matching lines @@
   DevToolsClientHost* client_host = manager->GetDevToolsClientHostFor(agent);
   DevToolsWindow* window = AsDevToolsWindow(client_host);
   if (!window || !window->is_docked())
     return NULL;
   return window->tab_contents();
 }
 
 // static
 bool DevToolsWindow::IsDevToolsWindow(RenderViewHost* window_rvh) {
   if (g_instances == NULL)
-    return NULL;
+    return false;
   DevToolsWindowList& instances = g_instances.Get();
   for (DevToolsWindowList::iterator it = instances.begin();
        it != instances.end(); ++it) {
     if ((*it)->tab_contents_->web_contents()->GetRenderViewHost() == window_rvh)
       return true;
   }
   return false;
 }
 
 // static
@@ skipping 639 matching lines @@
   CallClientFunction("InspectorFrontendAPI.appendedToURL", &url_value);
 }
 
 content::JavaScriptDialogCreator* DevToolsWindow::GetJavaScriptDialogCreator() {
   if (inspected_tab_ && inspected_tab_->web_contents()->GetDelegate()) {
     return inspected_tab_->web_contents()->GetDelegate()->
         GetJavaScriptDialogCreator();
   }
   return content::WebContentsDelegate::GetJavaScriptDialogCreator();
 }