Wipe out isolate_ pointer after disposing it.

This is to rule out the (unlikely) case that we're sending idle
notifications to a disposed isolate

BUG=425035
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/873723008

Cr-Commit-Position: refs/heads/master@{#315041}

gin/isolate_holder.cc

@@ -254,6 +254,7 @@ IsolateHolder::~IsolateHolder() {
 #endif
   isolate_data_.reset();
   isolate_->Dispose();
+  isolate_ = NULL;
 }
 
 // static