Add more fail safes to Extension Event Router.

To investigate a crash on ChromeOS, fail safe code and reporting are
added to EventRouter dispatcher.

Bug: 870838
Change-Id: I536a581c142d3b760f5a564507d4251b2b881c0e
Reviewed-on: https://chromium-review.googlesource.com/c/1356587Reviewed-by: Ramin Halavati <rhalavati@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Ramin Halavati <rhalavati@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612668}

extensions/browser/event_router.cc

@@ -730,12 +730,24 @@ void EventRouter::DoDispatchEventToSenderBookkeepingOnUI(
   }
   if (!ExtensionsBrowserClient::Get()->IsValidContext(browser_context))
     return;
+  // TODO(https://crbug.com/870838): Remove after investigating the bug.
+  if (!ExtensionRegistry::Get(browser_context)) {
+    LOG(ERROR) << "ExtensionRegistry does not exist.";
+    NOTREACHED();
+    return;
+  }
   const Extension* extension =
       ExtensionRegistry::Get(browser_context)->enabled_extensions().GetByID(
           extension_id);
   if (!extension)
     return;
   EventRouter* event_router = EventRouter::Get(browser_context);
+  // TODO(https://crbug.com/870838): Remove after investigating the bug.
+  if (!event_router) {
+    LOG(ERROR) << "EventRouter does not exist.";
+    NOTREACHED();
+    return;
+  }
   event_router->IncrementInFlightEvents(browser_context, extension, event_id,
                                         event_name);
   event_router->ReportEvent(histogram_value, extension,