Don't return base::nullopt for GesturePinch position.

One possible crash mechanism for this crash is if we return
base::nullopt when the touch_id == 0, cf.
DispatchTouchscreenGestureEvent().

Bug: 824774
Change-Id: I20e3c01bb4b8cbac8c5dbe3ad41d28a6b6843b78
Reviewed-on: https://chromium-review.googlesource.com/1024651Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: James MacLean <wjmaclean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552846}

content/browser/renderer_host/render_widget_host_input_event_router.cc

@@ -1047,7 +1047,7 @@ RenderWidgetHostInputEventRouter::FindTouchscreenGestureEventTarget(
   // target we could just return nullptr for pinch events, but since we know
   // where they are going we return the correct target.
   if (blink::WebInputEvent::IsPinchGestureEventType(gesture_event.GetType()))
-    return {root_view, false, base::nullopt};
+    return {root_view, false, gesture_event.PositionInWidget()};
 
   // Android sends gesture events that have no corresponding touch sequence, so
   // these we hit-test explicitly.