[in-place weak refs] Remove overzealous check.

We can allow right-trimming new space WeakFixedArrays at the end of mark
compact collection.

BUG=v8:7308

Change-Id: I1aa716b467952520f152c338f2042e023f6330ff
Reviewed-on: https://chromium-review.googlesource.com/1082198Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53471}

src/heap/heap.cc

@@ -2875,12 +2875,10 @@ void Heap::RightTrimFixedArray(FixedArrayBase* object, int elements_to_trim) {
 
 void Heap::RightTrimWeakFixedArray(WeakFixedArray* object,
                                    int elements_to_trim) {
-  // This function is safe to use only 1) during GC and 2) for old space
-  // WeakFixedArrays: 1) When marking, we record the weak slots, and shrinking
-  // invalidates them. 2) Scavenger might move new space WeakFixedArrays around,
-  // making the recorded slots collide with other objects.
+  // This function is safe to use only at the end of the mark compact
+  // collection: When marking, we record the weak slots, and shrinking
+  // invalidates them.
   DCHECK_EQ(gc_state(), MARK_COMPACT);
-  DCHECK(InOldSpace(object));
   CreateFillerForArray<WeakFixedArray>(object, elements_to_trim,
                                        elements_to_trim * kPointerSize);
 }