[fuzzer] Force a GC before shutting down isolates

We just deleted a persistent the line before, so make sure the context
is cleared up before disposing the isolate

R=ulan@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/1953253003
Cr-Commit-Position: refs/heads/master@{#36078}

test/fuzzer/fuzzer-support.cc

@@ -68,6 +68,7 @@ FuzzerSupport::~FuzzerSupport() {
     context_.Reset();
   }
 
+  isolate_->LowMemoryNotification();
   isolate_->Dispose();
   isolate_ = nullptr;