Instrument code entry slot recording to help with crash investigation.

BUG=chromium:414964
LOG=NO

Review-Url: https://codereview.chromium.org/1975593004
Cr-Commit-Position: refs/heads/master@{#36651}

src/heap/mark-compact.cc

@@ -3961,6 +3961,8 @@ void MarkCompactCollector::RecordCodeEntrySlot(HeapObject* host, Address slot,
   Page* source_page = Page::FromAddress(reinterpret_cast<Address>(host));
   if (target_page->IsEvacuationCandidate() &&
       !ShouldSkipEvacuationSlotRecording(host)) {
+    // TODO(ulan): remove this check after investigating crbug.com/414964.
+    CHECK(target->IsCode());
     RememberedSet<OLD_TO_OLD>::InsertTyped(source_page, CODE_ENTRY_SLOT, slot);
   }
 }