Sync core/timing/ interfaces with their specs, or lack thereof

Source/core/timing/PerformanceRenderTiming.idl

Index: Source/core/timing/PerformanceRenderTiming.idl
diff --git a/Source/core/timing/PerformanceRenderTiming.idl b/Source/core/timing/PerformanceRenderTiming.idl
index 57a8ba5c43f272afc4d2df599615b4ce286b8735..eafd49045873489c44196783016e56e850ac6dd8 100644
--- a/Source/core/timing/PerformanceRenderTiming.idl
+++ b/Source/core/timing/PerformanceRenderTiming.idl
@@ -28,9 +28,12 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-// See: http://w3c.github.io/frame-timing/
+// https://w3c.github.io/frame-timing/#performancerendertiming
+
 [
     RuntimeEnabled=FrameTimingSupport  // Experimental flag
 ] interface PerformanceRenderTiming : PerformanceEntry {
+    // TODO(philipj): This is named |sourceFrameNumber| in the spec.
     readonly attribute unsigned long sourceFrame;
+    // TODO(philipj): serializer = {inherit, attribute};
 };