[heap] Fix handling of AllocationSites in concurrent marking.

Now the concurrent marker skips the weak fields of AllocationSites.

BUG=chromium:694255

Change-Id: If920b6a14fa12c3a90ab0d283e22988ccbd48c0e
Reviewed-on: https://chromium-review.googlesource.com/574907
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46720}

src/heap/concurrent-marking.cc

@@ -151,6 +151,14 @@ class ConcurrentMarkingVisitor final
     return 0;
   }
 
+  int VisitAllocationSite(Map* map, AllocationSite* object) {
+    if (!ShouldVisit(object)) return 0;
+    int size = AllocationSite::BodyDescriptorWeak::SizeOf(map, object);
+    VisitMapPointer(object, object->map_slot());
+    AllocationSite::BodyDescriptorWeak::IterateBody(object, size, this);
+    return size;
+  }
+
   int VisitJSFunction(Map* map, JSFunction* object) {
     if (!ShouldVisit(object)) return 0;
     int size = JSFunction::BodyDescriptorWeak::SizeOf(map, object);