[Extensions + Net] Add .mjs to MIME type mappings

Add .mjs to type mappings for the javascript MIME type. This is
necessary for .mjs to be recognized as a valid for use in modules
when served by the extension protocols, where the mime type is
determined by our net code (as opposed to a server).

Update the extension API test for modules to include a .mjs file test.

Bug: 857076
Change-Id: Ib14cabbc52057cfb57c933de5a9c3ed4b13b87d2
Reviewed-on: https://chromium-review.googlesource.com/1117203Reviewed-by: Asanka Herath <asanka@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571921}

chrome/test/data/extensions/api_test/modules/background.js

@@ -5,6 +5,14 @@
 // TODO(devlin): Sadly, there's no good way of checking that the import
 // succeeds, so in the failure case the test just times out, rather than
 // giving a useful error.
-import {pass} from '/module.js';
+import {pass as jsPass} from '/module.js';
+import {pass as mjsPass} from '/module.mjs';
 
-pass();
+chrome.test.runTests([
+  function jsModule() {
+    jsPass();
+  },
+  function mjsModule() {
+    mjsPass();
+  },
+]);

chrome/test/data/extensions/api_test/modules/module.js

@@ -3,5 +3,5 @@
 // found in the LICENSE file.
 
 export function pass() {
-  chrome.test.notifyPass();
+  chrome.test.succeed();
 }

chrome/test/data/extensions/api_test/modules/module.mjs

+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+export function pass() {
+  chrome.test.succeed();
+}

net/base/mime_util.cc

@@ -97,7 +97,7 @@ static const MimeInfo kPrimaryMappings[] = {
     {"multipart/related", "mht,mhtml"},
     {"text/css", "css"},
     {"text/html", "html,htm,shtml,shtm"},
-    {"text/javascript", "js"},
+    {"text/javascript", "js,mjs"},
     {"text/xml", "xml"},
     {"video/mp4", "mp4,m4v"},
     {"video/ogg", "ogv,ogm"},