Fix LayoutTests that assume canplay, playing, and canplaythrough will only fire once.

LayoutTests/media/video-seek-by-small-increment.html

Index: LayoutTests/media/video-seek-by-small-increment.html
diff --git a/LayoutTests/media/video-seek-by-small-increment.html b/LayoutTests/media/video-seek-by-small-increment.html
index 12aa8ddcb830e27ed0b401427deecb7a4473a9ed..a9ec35294d47af7b55730d3ddebf2eee5c8ac2ed 100644
--- a/LayoutTests/media/video-seek-by-small-increment.html
+++ b/LayoutTests/media/video-seek-by-small-increment.html
@@ -42,7 +42,7 @@
                 findMediaElement();
                 video.src = findMediaFile("video", "content/test");
 
-                waitForEvent('canplaythrough', attemptSeek);
+                waitForEventOnce('canplaythrough', attemptSeek);
                 waitForEvent('seeked', seeked);
                 waitForEvent('seeking');
                 waitForEvent('play');