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

test/fuzzer/json.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 "test/fuzzer/fuzzer-support.h"
 
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
   v8_fuzzer::FuzzerSupport* support = v8_fuzzer::FuzzerSupport::Get();
   v8::Isolate* isolate = support->GetIsolate();
 
   v8::Isolate::Scope isolate_scope(isolate);
   v8::HandleScope handle_scope(isolate);
   v8::Context::Scope context_scope(support->GetContext());
   v8::TryCatch try_catch(isolate);
 
   if (size > INT_MAX) return 0;
   v8::Local<v8::String> source;
   if (!v8::String::NewFromOneByte(isolate, data, v8::NewStringType::kNormal,
                                   static_cast<int>(size))
            .ToLocal(&source)) {
     return 0;
   }
 
   v8::JSON::Parse(support->GetContext(), source).IsEmpty();
+  isolate->RequestGarbageCollectionForTesting(
+      v8::Isolate::kFullGarbageCollection);
   return 0;
 }