[heap] Update a check in incremental marker for Contexts

which are no longer derived from FixedArray and therefore IsFixedArray()
check no longer includes Contexts.

Bug: chromium:908877
Change-Id: I3aed0d38f5b1c00c9e27b7d5b6d29cdd5666ba86
Reviewed-on: https://chromium-review.googlesource.com/c/1352280Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57883}

src/heap/incremental-marking.cc

@@ -765,7 +765,8 @@ int IncrementalMarking::VisitObject(Map map, HeapObject* obj) {
     //    unsafe layout change.
     // 4. The object is materizalized by the deoptimizer.
     DCHECK(obj->IsHashTable() || obj->IsPropertyArray() ||
-           obj->IsFixedArray() || obj->IsJSObject() || obj->IsString());
+           obj->IsFixedArray() || obj->IsContext() || obj->IsJSObject() ||
+           obj->IsString());
   }
   DCHECK(marking_state()->IsBlack(obj));
   WhiteToGreyAndPush(map);