Check for NULL frame tree root.

content/browser/web_contents/web_contents_impl.cc

Index: content/browser/web_contents/web_contents_impl.cc
diff --git a/content/browser/web_contents/web_contents_impl.cc b/content/browser/web_contents/web_contents_impl.cc
index 81c00b1d78d4ff6ec6eabbc3a8f2235c00ed1554..8cb327c11120cfbc4bbc4b5b185e12f60a7172ef 100644
--- a/content/browser/web_contents/web_contents_impl.cc
+++ b/content/browser/web_contents/web_contents_impl.cc
@@ -2485,6 +2485,9 @@ void WebContentsImpl::OnUpdateFaviconURL(
 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()) {