Update webxr timestamp test to not compare against window.rAF()

The exact desired behavior is unclear, see immersive-web/webxr#943 and immersive-web/webxr#225

tests/wpt/metadata/MANIFEST.json

    "testharness"
   ],
   "webxr/xrSession_requestAnimationFrame_timestamp.https.html": [
-   "2796be9987b5f23e92d5f114fdaa972a5323a37c",
+   "8f5c21cba4015da10b9031f1cf100138e958c46e",
    "testharness"
   ],
   "webxr/xrSession_requestReferenceSpace.https.html": [

tests/wpt/web-platform-tests/webxr/xrSession_requestAnimationFrame_timestamp.https.html

@@ -20,25 +20,23 @@
       let counter = 0;
       let windowFrameTime = 0;
       let frameTime = 0;
+      let lastFrameTime = 0;
+      let firstFrame = true;
 
       function onFrameFirst(time, xrFrame) {
+        lastFrameTime = frameTime;
         frameTime = time;
+        firstFrame = false;
         let now = performance.now();
 
         t.step( () => {
           // This callback must be the first one called.
           assert_equals(counter, 0);
 
-          // window.requestAnimationFrame and session.requestAnimationFrame
-          // should be providing timestamps that are on the same scale and
-          // within a resonable margin of error of eachother. This means that
-          // this frame's timestamp should be larger than one provided to a
-          // previous window.requestAnimationFrame and should also be within
-          // a sane delta of it. One minute is probably overly generous here,
-          // but it will at least catch cases where the times are reported with
-          // entirely different bases.
-          assert_greater_than(frameTime, windowFrameTime);
-          assert_approx_equals(frameTime, windowFrameTime, ONE_MINUTE);
+          if (!firstFrame) {
+            assert_greater_than(frameTime, lastFrameTime);
+            assert_approx_equals(frameTime, lastFrameTime, TEN_SECONDS);
+          }
 
           // There's going to be some disparity between performance.now() and
           // the timestamp passed into the callback, but it shouldn't be huge.
@@ -47,6 +45,12 @@
           assert_approx_equals(frameTime, now, TEN_SECONDS);
         });
 
+        if (firstFrame) {
+          session.requestAnimationFrame(onFrameFirst);
+        } else {
+          resolve();
+        }
+
         counter++;
       }
 
@@ -65,21 +69,14 @@
           // Make sure all the previous callbacks fired as expected.
           assert_equals(counter, 11);
         });
-
-        // Finished.
-        resolve();
       }
 
-      window.requestAnimationFrame((time) => {
-        windowFrameTime = time;
-
-        // Queue up several callbacks
-        session.requestAnimationFrame(onFrameFirst);
-        for (let i = 0; i < 10; ++i) {
-          session.requestAnimationFrame(onFrameSubsequent);
-        }
-        session.requestAnimationFrame(onFrameLast);
-      });
+      session.requestAnimationFrame(onFrameFirst);
+      // Queue up several callbacks
+      for (let i = 0; i < 10; ++i) {
+        session.requestAnimationFrame(onFrameSubsequent);
+      }
+      session.requestAnimationFrame(onFrameLast);
 
     }));
 };