Fix preload tests failing on wpt.fyi dashboard, second attempt

https://crrev.com/c/1411963 fixed the url conflicts with other tests,
but the test still flakes when running repeatedly, because the resource
is loaded from disk cache in subsequent runs
(https://github.com/web-platform-tests/wpt/pull/14882).

We don't care if the preloaded resources came from cache in these tests,
so let's just verify the number of Resource Timing entries.

Bug: 922343
Change-Id: I2c1d146856a125dbbb641d7efb4e6151094f6a3e
Reviewed-on: https://chromium-review.googlesource.com/c/1436777Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626019}

third_party/blink/web_tests/external/wpt/preload/link-header-preload-nonce.html

@@ -9,8 +9,8 @@
 <script nonce="abc">
     window.addEventListener('load', t.step_func(function() {
         verifyPreloadAndRTSupport();
-        verifyNumberOfDownloads("resources/dummy.js?from-header&without-nonce", 0);
-        verifyNumberOfDownloads("resources/dummy.js?from-header&with-nonce", 1);
+        verifyNumberOfResourceTimingEntries("resources/dummy.js?from-header&without-nonce", 0);
+        verifyNumberOfResourceTimingEntries("resources/dummy.js?from-header&with-nonce", 1);
         t.done();
     }));
 </script>

third_party/blink/web_tests/external/wpt/preload/link-header-preload-srcset.tentative.html

@@ -11,16 +11,16 @@
 <script>
     window.addEventListener("load", t.step_func(function() {
         verifyPreloadAndRTSupport();
-        verifyNumberOfDownloads('resources/square.png?from-header&1x', 1);
-        verifyNumberOfDownloads('resources/square.png?from-header&2x', 0);
-        verifyNumberOfDownloads('resources/square.png?from-header&3x', 0);
-        verifyNumberOfDownloads('resources/square.png?from-header&base', 0);
-        verifyNumberOfDownloads('resources/square.png?from-header&200', 0);
-        verifyNumberOfDownloads('resources/square.png?from-header&400', 1);
-        verifyNumberOfDownloads('resources/square.png?from-header&800', 0);
-        verifyNumberOfDownloads('resources/square.png?from-header&150', 0);
-        verifyNumberOfDownloads('resources/square.png?from-header&300', 1);
-        verifyNumberOfDownloads('resources/square.png?from-header&600', 0);
+        verifyNumberOfResourceTimingEntries('resources/square.png?from-header&1x', 1);
+        verifyNumberOfResourceTimingEntries('resources/square.png?from-header&2x', 0);
+        verifyNumberOfResourceTimingEntries('resources/square.png?from-header&3x', 0);
+        verifyNumberOfResourceTimingEntries('resources/square.png?from-header&base', 0);
+        verifyNumberOfResourceTimingEntries('resources/square.png?from-header&200', 0);
+        verifyNumberOfResourceTimingEntries('resources/square.png?from-header&400', 1);
+        verifyNumberOfResourceTimingEntries('resources/square.png?from-header&800', 0);
+        verifyNumberOfResourceTimingEntries('resources/square.png?from-header&150', 0);
+        verifyNumberOfResourceTimingEntries('resources/square.png?from-header&300', 1);
+        verifyNumberOfResourceTimingEntries('resources/square.png?from-header&600', 0);
         t.done();
     }));
 </script>

third_party/blink/web_tests/external/wpt/preload/resources/preload_helper.js

@@ -20,3 +20,9 @@ function verifyNumberOfDownloads(url, number)
     });
     assert_equals(numDownloads, number, url);
 }
+
+function verifyNumberOfResourceTimingEntries(url, number)
+{
+    var numEntries = performance.getEntriesByName(getAbsoluteURL(url)).length;
+    assert_equals(numEntries, number, url);
+}