service worker: Initialize response type to kDefault.

Follow-up to r777235. Aligns with blink::ResourceResponse. It's
impractical to audit all network::mojom::URLResponseHead::New() sites
but I expect it's safe.

It looks like the default changed around Aug 2019 with these CLs:
- https://chromium-review.googlesource.com/c/chromium/src/+/1742782
- https://chromium-review.googlesource.com/c/chromium/src/+/1953671

Change-Id: If179612f23f0f4dbbaa1c2db2da9ebc1bb092e89
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2387038Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804644}

services/network/public/mojom/url_response_head.mojom

@@ -132,13 +132,7 @@ struct URLResponseHead {
   array<url.mojom.Url> url_list_via_service_worker;
 
   // https://fetch.spec.whatwg.org/#concept-response-type
-  //
-  // TODO(falken): Initialize to kDefault per spec, if that doesn't
-  // break things. Previously this field was not being initialized
-  // explicitly, so it was zero-initializing which happens to be the
-  // kBasic type. So changing it to kDefault could potentially change
-  // behavior.
-  FetchResponseType response_type = FetchResponseType.kBasic;
+  FetchResponseType response_type = FetchResponseType.kDefault;
 
   // The cache name of the CacheStorage from where the response is served via
   // the ServiceWorker. Empty if the response isn't from the CacheStorage.