Deflake video-colorspace* tests

This CL uses video.requestVideoFrameCallback to guarantee that a frame
is displayed on screen before ending the test.

Bug: 1099336
Change-Id: I02fa119bb20875111187da94460263a3d75e39e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2561681
Auto-Submit: Thomas Guilbert <tguilbert@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#831956}

third_party/blink/web_tests/media/video-colorspace-yuv420.html

@@ -7,7 +7,6 @@
     <p>Test correct colorspace for yuv420, i.e. YU12 video </p>
     <video/>
     <script>
-        setSrcByTagName('video', 'content/test_yuv420.ogv');
-        init();
+        setVideoSrcAndWaitForFirstFrame('content/test_yuv420.ogv');
     </script>
 </body>

third_party/blink/web_tests/media/video-colorspace-yuv422.html

@@ -7,7 +7,6 @@
     <p>Test correct colorspace for yuv422, i.e. YU16 video </p>
     <video/>
     <script>
-        setSrcByTagName('video', 'content/test_yuv422.ogv');
-        init();
+        setVideoSrcAndWaitForFirstFrame('content/test_yuv422.ogv');
     </script>
 </body>