Fix clusterfuzz crash on MojoInputMessages

The callsites were updated in
https://chromium-review.googlesource.com/c/chromium/src/+/793971 however
I left the DCHECK still in and didn't allow nullptr as the return value.
This caused the clusterfuzz failures.

BUG=789926,789906

Change-Id: I950f4ea72225faa0f9be3afbe022e0152079aa92
Reviewed-on: https://chromium-review.googlesource.com/801637Reviewed-by: Mustaq Ahmed <mustaq@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520685}

content/renderer/input/widget_input_handler_manager.cc

@@ -217,8 +217,9 @@ mojom::WidgetInputHandlerHost*
 WidgetInputHandlerManager::GetWidgetInputHandlerHost() {
   if (associated_host_)
     return associated_host_.get()->get();
-  DCHECK(host_);
-  return host_.get()->get();
+  if (host_)
+    return host_.get()->get();
+  return nullptr;
 }
 
 void WidgetInputHandlerManager::ObserveGestureEventOnMainThread(