Detach v8::Context from Worker and Worklet in its shutdown process

v8::Context may refer C++ object embedded by Blink, and it was left
stale on Worker and Worklet shutdown.
This CL ensures them to be cleared at the end of the shutdown process.

Bug: 804661
Change-Id: Ib66d1bf6bc88370310d65b04210ca763474ff8a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1578518
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653526}

third_party/blink/renderer/bindings/core/v8/worker_or_worklet_script_controller.cc

@@ -130,6 +130,13 @@ void WorkerOrWorkletScriptController::DisposeContextIfNeeded() {
                                      script_state_->GetContext());
   }
 
+  {
+    ScriptState::Scope scope(script_state_);
+    // This detaches v8::MicrotaskQueue pointer from v8::Context, so that we can
+    // destroy EventLoop safely.
+    script_state_->GetContext()->DetachGlobal();
+  }
+
   script_state_->DisposePerContextData();
   script_state_->DissociateContext();
 }