Move promise_test and assert_promise_rejects out of worker-test-harness.

LayoutTests/http/tests/serviceworker/resources/response-content-worker.js

-importScripts('worker-test-harness.js');
+importScripts('worker-testharness.js');
+importScripts('../../resources/testharness-helpers.js');
 
 promise_test(function() {
     var response = new Response('test string');
     assert_equals(
       response.headers.get('Content-Type'),
       'text/plain;charset=UTF-8',
       'A Response constructed with a string should have a Content-Type.');
     return response.text()
       .then(function(text) {
           assert_equals(text, 'test string',
@@ skipping 79 matching lines @@
           response3 = response.clone();
           assert_true(response3.bodyUsed,
                       "bodyUsed should be true in clone of consumed response.");
           return response2.text();
         })
       .then(function(text) {
           assert_equals(text, 'test string',
             'Response clone response body text should match.');
         });
   }, 'Behavior of bodyUsed in Response and clone behavior.');