Fix crash in event sender

This is a small amendment to https://crrev.com/c/1763375. The null check
added there should go before the DCHECK.

Bug: 997430
Change-Id: Iabdef43de616b6207516b7efff1c6ecd8e81de81
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1771621
Auto-Submit: David Bokan <bokan@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690986}

content/shell/test_runner/event_sender.cc

@@ -2844,11 +2844,11 @@ blink::WebWidget* EventSender::widget() {
 }
 
 blink::WebFrameWidget* EventSender::mainFrameWidget() {
+  if (!view() || !view()->MainFrame())
+    return nullptr;
   DCHECK(view()->MainFrame()->IsWebLocalFrame())
       << "Event Sender doesn't support being run in a remote frame for this "
          "operation.";
-  if (!view() || !view()->MainFrame())
-    return nullptr;
   return view()->MainFrame()->ToWebLocalFrame()->FrameWidget();
 }