Fix crash in AXEventGenerator::PostprocessEvents

This is a quick fix to merge. I'm going to follow up with a patch to
explore the underlying cause.

Bug: 1135103
Change-Id: I7f5f7a63b34e7391873755f0917ac6b971e8b0aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2459008Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Abigail Klein <abigailbklein@google.com>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815160}

ui/accessibility/ax_event_generator.cc

@@ -877,6 +877,12 @@ void AXEventGenerator::PostprocessEvents() {
   // First pass through |tree_events_|, remove events that we do not need.
   for (auto& iter : tree_events_) {
     AXNode* node = iter.first;
+
+    // TODO(http://crbug.com/2279799): remove all of the cases that could
+    // add a null node to |tree_events|.
+    if (!node)
+      continue;
+
     std::set<EventParams>& node_events = iter.second;
 
     // A newly created live region or alert should not *also* fire a