Fix wpt/beacon/headers/header-content-type.html slowness

This CL uses ResourceTiming API to speed up the test cases while
avoiding flakiness.

Bug: 779965
Change-Id: I40dc2fae470df296c87c12e432fc7fc7644cc40a

beacon/headers/header-content-type.html

@@ -10,36 +10,30 @@
     <script src="/common/utils.js"></script>
     <script src="/common/get-host-info.sub.js"></script>
     <script>
-var RESOURCES_DIR = "/beacon/resources/";
-
-function pollResult(test, id) {
-  var checkUrl = RESOURCES_DIR + "content-type.py?cmd=get&id=" + id;
-
-  return new Promise(resolve => {
-    step_timeout(test.step_func(() => {
-      fetch(checkUrl).then(response => {
-        response.text().then(body => {
-          resolve(body);
-        });
-      });
-    }), 1000);
-  });
-}
+const RESOURCES_DIR = "/beacon/resources/";
 
 function testContentTypeHeader(what, contentType, title) {
-  var testBase = RESOURCES_DIR;
-  var id = self.token();
-  var testUrl = testBase + "content-type.py?cmd=put&id=" + id;
-
-  promise_test(function(test) {
+  function wait(ms) {
+    return new Promise(resolve => step_timeout(resolve, ms));
+  }
+  promise_test(async t => {
+    const id = self.token();
+    const testUrl = new Request(RESOURCES_DIR + "content-type.py?cmd=put&id=" + id).url;
     assert_true(navigator.sendBeacon(testUrl, what), "SendBeacon Succeeded");
-    return pollResult(test, id) .then(result => {
-      if (contentType == "multipart/form-data") {
-        assert_true(result.startsWith(contentType), "Correct Content-Type header result");
-      } else {
-        assert_equals(result, contentType, "Correct Content-Type header result");
-      }
-    });
+    assert_equals(performance.getEntriesByName(testUrl).length, 0);
+
+    do {
+      await wait(50);
+    } while (performance.getEntriesByName(testUrl).length === 0);
+    assert_equals(performance.getEntriesByName(testUrl).length, 1);
+    const checkUrl = RESOURCES_DIR + "content-type.py?cmd=get&id=" + id;
+    const response = await fetch(checkUrl);
+    const text = await response.text();
+    if (contentType == "multipart/form-data") {
+      assert_true(text.startsWith(contentType), "Correct Content-Type header result");
+    } else {
+      assert_equals(text, contentType, "Correct Content-Type header result");
+    };
   }, "Test content-type header for a body " + title);
 }