Crash when SetFocusedElement is called during layout

I think the crash in https://crbug.com/848212 is caused due to
Document::focused_element_ being cleared out during the
UpdateStyleAndLayoutIgnorePendingStylesheetsForNode call in
Document::SetFocusedElement, but I'm unable to reproduce a situtation
where this happens. This CL adds a CHECK that crashes when
SetFocusedElement is called during UpdateStyleAndLayout, just so I can
get some stack traces in the wild and confirm this theory.

Bug: 848212
Change-Id: Ia9293ea155e4ae976592166e7ab4f936f25f3f38
Reviewed-on: https://chromium-review.googlesource.com/1085607
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564337}

third_party/blink/renderer/core/dom/document.cc

@@ -350,6 +350,18 @@ class DocumentOutliveTimeReporter : public BlinkGCObserver {
   int gc_age_when_document_detached_ = 0;
 };
 
+class SetFocusedElementForbiddenScope {
+ public:
+  SetFocusedElementForbiddenScope() { count_++; }
+  ~SetFocusedElementForbiddenScope() { count_--; }
+  static bool IsSetFocusedElementForbidden() { return count_ > 0; }
+
+ private:
+  static int count_;
+};
+
+int SetFocusedElementForbiddenScope::count_ = 0;
+
 static const unsigned kCMaxWriteRecursionDepth = 21;
 
 // This amount of time must have elapsed before we will even consider scheduling
@@ -2348,6 +2360,7 @@ void Document::UpdateStyleAndLayoutTreeForNode(const Node* node) {
 
 void Document::UpdateStyleAndLayoutIgnorePendingStylesheetsForNode(Node* node) {
   DCHECK(node);
+  SetFocusedElementForbiddenScope set_focused_element_scope;
   if (!node->InActiveDocument())
     return;
   UpdateStyleAndLayoutIgnorePendingStylesheets();
@@ -4506,6 +4519,10 @@ bool Document::SetFocusedElement(Element* new_focused_element,
                                  const FocusParams& params) {
   DCHECK(!lifecycle_.InDetach());
 
+  // TODO(adithyas): This CHECK has been added to investigate
+  // https://crbug.com/848212, remove this after bug is resolved.
+  CHECK(!SetFocusedElementForbiddenScope::IsSetFocusedElementForbidden());
+
   clear_focused_element_timer_.Stop();
 
   // Make sure newFocusedNode is actually in this document