Fix media layout tests that made invalid assumptions about timeupdate.

LayoutTests/media/media-controller-playbackrate.html

Index: LayoutTests/media/media-controller-playbackrate.html
diff --git a/LayoutTests/media/media-controller-playbackrate.html b/LayoutTests/media/media-controller-playbackrate.html
index e5a142b44a91494a8e26532d624c85a48e3cda43..3ea2ed562e1599ee5b7c41429f21ee91e6d88d32 100644
--- a/LayoutTests/media/media-controller-playbackrate.html
+++ b/LayoutTests/media/media-controller-playbackrate.html
@@ -26,6 +26,13 @@
                 video.defaultPlaybackRate = 0;
                 video.controller.playbackRate = 0;
                 video.controller.defaultPlaybackRate = 0;
+
+                // Wait for the seek initiated by the "bringing up to speed"
+                // step to complete.
+                waitForEventOnce('seeked', seeked);
+            };
+
+            var seeked = function() {
                 waitForEvent('play', play);
                 video.play();
             };