[ServiceWorker] Fix the timing of state transition in NewScriptLoader

ServiceWorkerNewScriptLoader uses an internal variable called
|network_loader_state_| to control the state of URLLoaderClient. This
state is changed from kLoadingHeader to kWaitingForBody at the end of
OnReceiveResponse(). However, WriteHeaders() could call
OnWriteHeadersComplete() synchronously and it calls
MaybeStartNetworkConsumerHandleWatcher() when |network_loader_state_| is set to
kLoadingHeader. And later OnStartLoadingResponseBody() calls
MaybeStartNetworkConsumerHandleWatcher() again and hits
DCHECK_EQ(kNotStarted, body_writer_state_).

Change-Id: I3b9450ca53e6b98423d532ea02134d002d7753b6
Reviewed-on: https://chromium-review.googlesource.com/1237634Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Commit-Queue: Momoko Hattori <momohatt@google.com>
Cr-Commit-Position: refs/heads/master@{#593809}

content/browser/service_worker/service_worker_new_script_loader.cc

@@ -248,6 +248,8 @@ void ServiceWorkerNewScriptLoader::OnReceiveResponse(
     version_->SetMainScriptHttpResponseInfo(*response_info);
   }
 
+  network_loader_state_ = NetworkLoaderState::kWaitingForBody;
+
   WriteHeaders(
       base::MakeRefCounted<HttpResponseInfoIOBuffer>(std::move(response_info)));
 
@@ -259,11 +261,9 @@ void ServiceWorkerNewScriptLoader::OnReceiveResponse(
     network::ResourceResponseHead new_response_head = response_head;
     new_response_head.ssl_info.reset();
     client_->OnReceiveResponse(new_response_head);
-  } else {
-    client_->OnReceiveResponse(response_head);
+    return;
   }
-
-  network_loader_state_ = NetworkLoaderState::kWaitingForBody;
+  client_->OnReceiveResponse(response_head);
 }
 
 void ServiceWorkerNewScriptLoader::OnReceiveRedirect(