Check for NULL frame tree root.

content/browser/web_contents/web_contents_impl.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 "content/browser/web_contents/web_contents_impl.h"
 
 #include <utility>
 
 #include "base/command_line.h"
 #include "base/metrics/histogram.h"
@@ skipping 2467 matching lines @@
 void WebContentsImpl::OnUpdateFaviconURL(
     int32 page_id,
     const std::vector<FaviconURL>& candidates) {
   FOR_EACH_OBSERVER(WebContentsObserver, observers_,
                     DidUpdateFaviconURL(page_id, candidates));
 }
 
 FrameTreeNode* WebContentsImpl::FindFrameTreeNodeByID(int64 frame_id) {
   FrameTreeNode* node = NULL;
   std::queue<FrameTreeNode*> queue;
+
+  DCHECK(frame_tree_root_.get()) <<

[Charlie Reis, 2013/06/04 00:00:55]

DCHECKs only get hit during debug builds, so I don't expect many people are
going to hit this.

+      GetLastCommittedURL() << "|" << GetActiveURL();

[Charlie Reis, 2013/06/04 00:00:55]

These wouldn't show up in a crash dump, so at best we would ask for someone to
paste them into a bug report from the console.

I'm also not sure the URL tells us much.  The URL seems orthogonal to how we get
into a state with a null root-- wouldn't it be more about timing of how the tab
is opened and closed?

   queue.push(frame_tree_root_.get());
 
   while (!queue.empty()) {
     node = queue.front();
     queue.pop();
     if (node->frame_id() == frame_id)
       return node;
 
     for (size_t i = 0; i < node->child_count(); ++i)
       queue.push(node->child_at(i));
@@ skipping 1081 matching lines @@
 }
 
 BrowserPluginGuestManager*
     WebContentsImpl::GetBrowserPluginGuestManager() const {
   return static_cast<BrowserPluginGuestManager*>(
       GetBrowserContext()->GetUserData(
           browser_plugin::kBrowserPluginGuestManagerKeyName));
 }
 
 }  // namespace content