Correctly handle FindNext from the Find dialog.

chrome_frame/chrome_frame_automation.cc

Index: chrome_frame/chrome_frame_automation.cc
diff --git a/chrome_frame/chrome_frame_automation.cc b/chrome_frame/chrome_frame_automation.cc
index 147cdeefb285aaa5ba000b6de5457a596f5c6d72..b4d302300e173ac3eac986f366b014c1a89ed6b6 100644
--- a/chrome_frame/chrome_frame_automation.cc
+++ b/chrome_frame/chrome_frame_automation.cc
@@ -783,7 +783,9 @@ void ChromeFrameAutomationClient::FindInPage(const std::wstring& search_string,
                                              FindInPageDirection forward,
                                              FindInPageCase match_case,
                                              bool find_next) {
-  DCHECK(tab_.get());
+  // Note that we can be called by the find dialog after the tab has gone away.
+  if (!tab_)
+    return;
 
   // What follows is quite similar to TabProxy::FindInPage() but uses
   // the SyncMessageReplyDispatcher to avoid concerns about blocking