Enforce RequestInit enum values

This makes the Fetch API more standards compliant by
enforcing the enum values in the RequestInit dictionary,
as this is not taken care of by the IDL bindings themselves.
This enforces the mode, credentials, cache, and redirect properties
of RequestInit.

R=yhirano@chromium.org, yoav@yoav.ws

Bug: 777116
Change-Id: I8db5fd991b5ffa039dc2f67b4d8f525647fbf915
Reviewed-on: https://chromium-review.googlesource.com/841270
Commit-Queue: Yoav Weiss <yoav@yoav.ws>
Reviewed-by: Yoav Weiss <yoav@yoav.ws>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541515}

service-workers/service-worker/webvtt-cross-origin.https.html

@@ -95,9 +95,9 @@
     url += '?url=' + get_host_info().HTTPS_REMOTE_ORIGIN + '/media/foo.vtt';
     // Add '&mode' to tell the service worker to do a CORS request.
     url += '&mode=cors';
-    // Add '&credentials=anonymous' to allow Access-Control-Allow-Origin=* so
+    // Add '&credentials=same-origin' to allow Access-Control-Allow-Origin=* so
     // that CORS will succeed if the service approves it.
-    url += '&credentials=anonymous';
+    url += '&credentials=same-origin';
     return load_track(url)
       .then(result => {
           assert_equals(result, 'load event');
@@ -142,9 +142,9 @@
     url += '?url=' + encodeURIComponent(redirector_url + encodeURIComponent(redirect_target));
     // Add '&mode' to tell the service worker to do a CORS request.
     url += '&mode=cors';
-    // Add '&credentials=anonymous' to allow Access-Control-Allow-Origin=* so
+    // Add '&credentials=same-origin' to allow Access-Control-Allow-Origin=* so
     // that CORS will succeed if the server approves it.
-    url += '&credentials=anonymous';
+    url += '&credentials=same-origin';
     return load_track(url)
       .then(result => {
           assert_equals(result, 'error event');
@@ -160,9 +160,9 @@
     url += '?url=' + encodeURIComponent(redirector_url + encodeURIComponent(redirect_target));
     // Add '&mode' to tell the service worker to do a CORS request.
     url += '&mode=cors';
-    // Add '&credentials=anonymous' to allow Access-Control-Allow-Origin=* so
+    // Add '&credentials=same-origin' to allow Access-Control-Allow-Origin=* so
     // that CORS will succeed if the server approves it.
-    url += '&credentials=anonymous';
+    url += '&credentials=same-origin';
     return load_track(url)
       .then(result => {
           assert_equals(result, 'load event');