Adds keyboard functionality for videos.

With this CL, in focus videos can be controlled using the up/down keys
(volume), left/right keys (scrubbing), and space/enter (play/pause). This
improves upon the existing functionality, where one must tab over to the
individual control to manipulate it. Instead, the video now only needs to be
in focus to be able to control it.

BUG=135661

Review-Url: https://codereview.chromium.org/2700663002
Cr-Commit-Position: refs/heads/master@{#475706}
9 files changed