Fix RenderWidgetHostInputEventRouter::RouteMouseWheelEvent crash on Mac.

https://chromium-review.googlesource.com/904547 hasn't fix the crashes.
This cl checks the validity of the event router before sending the wheel
end events.

Bug: 791963
Change-Id: I5af23c24e5dcd9a8351ba00a8fdcfde4e2109b7d
Reviewed-on: https://chromium-review.googlesource.com/964736Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Commit-Queue: Sahel Sharifymoghaddam <sahel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543704}

content/browser/renderer_host/input/mouse_wheel_phase_handler.cc

@@ -136,7 +136,7 @@ void MouseWheelPhaseHandler::SendSyntheticWheelEventWithPhaseEnded(
 
   if (should_route_event) {
     RenderWidgetHostImpl* host = host_view_->GetRenderWidgetHostImpl();
-    if (!host)
+    if (!host || !host->delegate() || !host->delegate()->GetInputEventRouter())
       return;
 
     host->delegate()->GetInputEventRouter()->RouteMouseWheelEvent(