Add output for unexpected event type.

Since the DCHECK failure in the associated bug is hard to reproduce
locally, add output to the DCHECK to determine what type(s) of event(s)
are causing it to fail. Hopefully this will get recorded in the crash
output should the issue reproduce.

Bug: 985944
Change-Id: I3fa8032fb3130d366248a5b134887f54d36070f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1724585Reviewed-by: Kevin McNee <mcnee@chromium.org>
Commit-Queue: James MacLean <wjmaclean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682255}

content/renderer/browser_plugin/browser_plugin.cc

@@ -588,7 +588,8 @@ blink::WebInputEventResult BrowserPlugin::HandleInputEvent(
     DCHECK(blink::WebInputEvent::kGestureTapDown == event.GetType() ||
            blink::WebInputEvent::kGestureScrollBegin == event.GetType() ||
            blink::WebInputEvent::kGestureScrollEnd == event.GetType() ||
-           gesture_event.resending_plugin_id == browser_plugin_instance_id_);
+           gesture_event.resending_plugin_id == browser_plugin_instance_id_)
+        << "Unexpected event seen: " << event.GetName(event.GetType());
 
     // We shouldn't be forwarding GestureEvents to the Guest anymore. Indicate
     // we handled this only if it's a non-resent event.