Add test for time resolution of User Timing

LayoutTests/http/tests/misc/webtiming-resolution.html

Index: LayoutTests/http/tests/misc/webtiming-resolution.html
diff --git a/LayoutTests/http/tests/misc/webtiming-resolution.html b/LayoutTests/http/tests/misc/webtiming-resolution.html
index ea0027180edea1a4d0c9e7e8003cb5fc4927199f..812af6acba83755431a922dd40d77bcf1ee770c7 100644
--- a/LayoutTests/http/tests/misc/webtiming-resolution.html
+++ b/LayoutTests/http/tests/misc/webtiming-resolution.html
@@ -8,17 +8,32 @@
 <script>
 description("Verifies the minimum resolution is 5 microseconds.");
 
-var t0 = performance.now();
-var t1 = performance.now();
-while (t0 == t1) {
-    t1 = performance.now();
+function testTimeResolution(highResTimeFunc) {
+    var t0 = highResTimeFunc();
+    var t1 = highResTimeFunc();
+    while (t0 == t1) {
+        t1 = highResTimeFunc();
+    }
+
+    var expectedResolutionMilliseconds = 0.005;
+    var integerMultipleOfResolution = (t1 - t0) / expectedResolutionMilliseconds;
+    shouldBeNearZeroOrOne = integerMultipleOfResolution % 1;
+    shouldBe("shouldBeNearZeroOrOne < 1e-10 || Math.abs(shouldBeNearZeroOrOne - 1) < 1e-10", "true");
+}
+
+function timeByPerformanceNow() {
+    return performance.now();
 }
 
-var expectedResolutionMilliseconds = 0.005;
-var integerMultipleOfResolution = (t1 - t0) / expectedResolutionMilliseconds;
-var shouldBeNearZeroOrOne = integerMultipleOfResolution % 1;
-shouldBe("shouldBeNearZeroOrOne < 1e-10 || Math.abs(shouldBeNearZeroOrOne - 1) < 1e-10", "true");
+function timeByUserTiming() {
+    performance.mark('timer');
+    var t = performance.getEntriesByName('timer')[0].startTime;
+    performance.clearMarks('timer');
+    return t;
+}
 
+testTimeResolution(timeByPerformanceNow);
+testTimeResolution(timeByUserTiming);
 </script>
 </body>
 </html>