[VizHitTesting] Fix an unchecked access to a optional value

target_location has a value only if the target exists. When it doesn't
we early exit the DispatchMouseEvent(). In addition, we should skip
setting the mouse_down_post_transformed_coordinate_.

Bug: 985692
Change-Id: I4cd4dad3b1caefaf2bd7e81660e257a4bdb5fdfa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715227
Commit-Queue: Yi Gu <yigu@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680119}

content/browser/renderer_host/render_widget_host_input_event_router.cc

@@ -1828,7 +1828,7 @@ void RenderWidgetHostInputEventRouter::DispatchEventToTarget(
   if (target && target->ScreenRectIsUnstableFor(event))
     event.SetTargetFrameMovedRecently();
   if (blink::WebInputEvent::IsMouseEventType(event.GetType())) {
-    if (event.GetType() == blink::WebInputEvent::kMouseDown) {
+    if (target && event.GetType() == blink::WebInputEvent::kMouseDown) {
       mouse_down_post_transformed_coordinate_.SetPoint(target_location->x(),
                                                        target_location->y());
       last_mouse_down_target_ = target;