Oilpan: Fix DCHECK for fixups_ in HeapCompaction

This CL changes DCHECK for fixups_ to Fixups().
They are guaranteed that AddCompactingPage() and RegisterMovingObjectCallback() are called before these method.
But we collect also HeapCollections that have no buffer so it is not guaranteed.

Bug: 864425
Change-Id: Id28cc98f060f4125bca04664a9e8416f9cf97fcc
Reviewed-on: https://chromium-review.googlesource.com/1156527Reviewed-by: Keishi Hattori <keishi@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Haruka Matsumura <harukamt@google.com>
Cr-Commit-Position: refs/heads/master@{#579692}

third_party/blink/renderer/platform/heap/heap_compact.cc

@@ -422,19 +422,17 @@ void HeapCompact::FinishedArenaCompaction(NormalPageArena* arena,
 }
 
 void HeapCompact::Relocate(Address from, Address to) {
-  DCHECK(fixups_);
-  fixups_->Relocate(from, to);
+  Fixups().Relocate(from, to);
 }
 
 void HeapCompact::StartThreadCompaction() {
   if (!do_compact_)
     return;
 
-  DCHECK(fixups_);
   // The mapping between the slots and the backing stores are created
   for (auto** slot : traced_slots_) {
     if (*slot)
-      fixups_->Add(slot);
+      Fixups().Add(slot);
   }
   traced_slots_.clear();
 }