Adds keyboard functionality for videos.

third_party/WebKit/LayoutTests/media/controls/controls-video-keynav.html

+<DOCTYPE html>
+<title>Test media controls video keyboard navigation</title>
+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+<script src="../media-file.js"></script>
+<script src="../media-controls.js"></script>
+<video controls></video>
+<script>
+test(_ => {
+  assert_true('eventSender' in window);
+}, 'eventSender required');
+
+async_test(function(t) {
+    var video = document.querySelector("video");
+    video.src = findMediaFile("video", "../content/test");
+    assert_equals(video.volume, 1);
+    assert_equals(video.currentTime, 0);
+
+    video.oncanplaythrough = t.step_func_done(function() {
+      // Focus the video.
+      video.focus();
+
+      // 'Enter' plays the video.
+      eventSender.keyDown("Enter");
+      assert_false(video.paused);
+
+      // 'Down' reduces volume.
+      eventSender.keyDown("ArrowDown");
+      assert_less_than(video.volume, 1);
+
+      // 'Up' increases volume.
+      eventSender.keyDown("ArrowUp");
+      assert_equals(video.volume, 1);
+
+      // 'Enter' again pauses the video.
+      eventSender.keyDown("Enter");
+      assert_true(video.paused);
+
+      // 'Space' also pauses/plays the video.
+      eventSender.keyDown(" ");
+      assert_false(video.paused);
+
+      eventSender.keyDown(" ");
+      assert_true(video.paused);
+
+      var cur = video.currentTime;
+
+      // 'Right' scrubs the timeline forward.
+      // Used to move the timeline off the first frame which is neccessary to
+      // test 'Home', otherwise 'Home' is a no-op.
+      eventSender.keyDown("ArrowRight");
+      assert_greater_than(video.currentTime, cur);
+
+      // 'Home' sets the timeline to 0.
+      eventSender.keyDown("Home");
+      assert_equals(video.currentTime, 0);
+
+      // 'End' sets the timeline to end.
+      eventSender.keyDown("End");
+      assert_equals(video.currentTime, video.duration);
+
+      // 'Left' scrubs the timeline back.
+      eventSender.keyDown("ArrowLeft");
+      assert_less_than(video.currentTime, video.duration);
+    });
+});
+</script>