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

Source/core/timing/PerformanceCompositeTiming.idl

Index: Source/core/timing/PerformanceCompositeTiming.idl
diff --git a/Source/core/timing/PerformanceCompositeTiming.idl b/Source/core/timing/PerformanceCompositeTiming.idl
index bf98922244984f3c494ffd21d09fc0ac267d96dd..60eb0754bb0dedb95e9edd8169acc7c3508a23e2 100644
--- a/Source/core/timing/PerformanceCompositeTiming.idl
+++ b/Source/core/timing/PerformanceCompositeTiming.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/#performancecompositetiming
+
 [
     RuntimeEnabled=FrameTimingSupport  // Experimental flag
 ] interface PerformanceCompositeTiming : PerformanceEntry {
+    // TODO(philipj): This is named |sourceFrameNumber| in the spec.
     readonly attribute unsigned long sourceFrame;
+    // TODO(philipj): serializer = {inherit, attribute};
 };