Add some NULL-checks for ExtensionHost::extension to fix a crash bug.

chrome/browser/extensions/extension_event_router.cc

Index: chrome/browser/extensions/extension_event_router.cc
diff --git a/chrome/browser/extensions/extension_event_router.cc b/chrome/browser/extensions/extension_event_router.cc
index ab072009a98978ba445a432cb022c3bb4e934f7c..f7050e4c441e3bec63f0830b3451cb4d426a308c 100644
--- a/chrome/browser/extensions/extension_event_router.cc
+++ b/chrome/browser/extensions/extension_event_router.cc
@@ -431,6 +431,9 @@ void ExtensionEventRouter::OnEventAck(
   // The event ACK is routed to the background host, so this should never be
   // NULL.
   CHECK(host);
+  if (!host->extension())
+    return;
+
   CHECK(host->extension()->has_lazy_background_page());
   pm->DecrementLazyKeepaliveCount(host->extension());
 }