[Background Fetch] Add back ScriptState::Scope

to DispatchBackgroundFetchedEvent.

Creation of the scope modifies V8 state, and associates scriptstate with
v8::Context. This helps in the using script state when it's needed later.

TBR=haraken@chromium.org

Bug: 836787
Change-Id: I7d0c2cb445d74f85d918d0435e6dac2511365ee6
Reviewed-on: https://chromium-review.googlesource.com/1027836
Commit-Queue: Anita Woodruff <awdf@chromium.org>
Reviewed-by: Anita Woodruff <awdf@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553564}

third_party/blink/renderer/modules/serviceworkers/service_worker_global_scope_proxy.cc

@@ -212,6 +212,9 @@ void ServiceWorkerGlobalScopeProxy::DispatchBackgroundFetchedEvent(
   ScriptState* script_state =
       WorkerGlobalScope()->ScriptController()->GetScriptState();
 
+  // Do not remove this, it modifies V8 state.
+  ScriptState::Scope scope(script_state);
+
   BackgroundFetchSettledEventInit init;
   init.setId(developer_id);
   init.setFetches(BackgroundFetchSettledFetches::Create(script_state, fetches));