Add GC request to libFuzzers in attempt to avoid parasitic coverage.

test/fuzzer/parser.cc

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <limits.h>
 #include <stddef.h>
 #include <stdint.h>
 
 #include "include/v8.h"
 #include "src/objects.h"
@@ skipping 20 matching lines @@
           v8::internal::Vector<const uint8_t>(data, static_cast<int>(size)));
   if (source.is_null()) return 0;
 
   v8::internal::Handle<v8::internal::Script> script =
       factory->NewScript(source.ToHandleChecked());
   v8::internal::Zone zone(i_isolate->allocator());
   v8::internal::ParseInfo info(&zone, script);
   info.set_global();
   v8::internal::Parser parser(&info);
   parser.Parse(&info);
+  isolate->RequestGarbageCollectionForTesting(
+      v8::Isolate::kFullGarbageCollection);
   return 0;
 }