[Backport] Security bug 1155297 (3/3)

Cherry-pick of patch originally reviewed on
https://chromium-review.googlesource.com/c/chromium/src/+/2838329:
M86-LTS: Add null pointer check in RenderWidgetHostInputEventRouter

We have some crashes in RenderWidgetHostInputEventRouter class, we are
adding some null pointer check in this class to avoid the crash.

(cherry picked from commit 5f47666b79ac7ded20e1c7657037498561bd3352)

Bug: 1155297
Change-Id: I3b63d5748523ae2ce8ab469832adfc75d586e411
Reviewed-by: Charlie Reis <[email protected]>
Commit-Queue: Lan Wei <[email protected]>
Cr-Original-Commit-Position: refs/heads/master@{#871108}
Reviewed-by: Lan Wei <[email protected]>
Commit-Queue: Achuith Bhandarkar <[email protected]>
Owners-Override: Achuith Bhandarkar <[email protected]>
Cr-Commit-Position: refs/branch-heads/4240@{#1617}
Cr-Branched-From: f297677702651916bbf65e59c0d4bbd4ce57d1ee-refs/heads/master@{#800218}
Reviewed-by: Allan Sandfeld Jensen <[email protected]>

Author: LanWei22

chromium/content/browser/renderer_host/render_widget_host_input_event_router.cc

@@ -1970,7 +1970,7 @@ void RenderWidgetHostInputEventRouter::OnAggregatedHitTestRegionListUpdated(
     const std::vector<viz::AggregatedHitTestRegion>& hit_test_data) {
   for (auto& region : hit_test_data) {
     auto iter = owner_map_.find(region.frame_sink_id);
-    if (iter != owner_map_.end())
+    if (iter != owner_map_.end() && iter->second)
       iter->second->NotifyHitTestRegionUpdated(region);
   }
 }