Add more checks to diagnose MediaQuerySet issue

third_party/WebKit/Source/core/dom/DocumentStyleSheetCollection.cpp

Index: third_party/WebKit/Source/core/dom/DocumentStyleSheetCollection.cpp
diff --git a/third_party/WebKit/Source/core/dom/DocumentStyleSheetCollection.cpp b/third_party/WebKit/Source/core/dom/DocumentStyleSheetCollection.cpp
index bb422e0ca97c250efaa68788420776827ce1b193..33c9cc9ca2d4f51517938796f1aef1f598847d1a 100644
--- a/third_party/WebKit/Source/core/dom/DocumentStyleSheetCollection.cpp
+++ b/third_party/WebKit/Source/core/dom/DocumentStyleSheetCollection.cpp
@@ -50,7 +50,12 @@ void DocumentStyleSheetCollection::CollectStyleSheetsFromCandidates(
     DocumentStyleSheetCollector& collector) {
   DocumentOrderedList::MutationForbiddenScope mutation_forbidden_(
       &style_sheet_candidate_nodes_);
+  // TODO(keishi) Check added for crbug.com/699269 diagnosis. Remove once done.
+  CHECK(HeapObjectHeader::FromPayload(this)->IsValid());
+  CHECK(ThreadState::Current()->IsOnThreadHeap(this));
   for (Node* n : style_sheet_candidate_nodes_) {
+    CHECK(HeapObjectHeader::FromPayload(n)->IsValid());
+    CHECK(ThreadState::Current()->IsOnThreadHeap(n));
     StyleSheetCandidate candidate(*n);
 
     DCHECK(!candidate.IsXSL());