[snapshot] Remove overzealous DISALLOW_GARBAGE_COLLECTION

As of https://chromium-review.googlesource.com/c/v8/v8/+/2452689 ,
ObjectCacheIndexMap uses IdentityMap which deals with GC gracefully.

Change-Id: I6d43ee2c1e330556b0ab7e4a6c313d5b37086343
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2742615Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73266}

src/snapshot/serializer.h

@@ -151,8 +151,6 @@ class ObjectCacheIndexMap {
   }
 
  private:
-  DISALLOW_GARBAGE_COLLECTION(no_gc_)
-
   IdentityMap<int, base::DefaultAllocationPolicy> map_;
   int next_index_;
 };