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- Add a WelcomeVideoController.pause() function
- Make WelcomeVideoController.play() call play on the active video, too
- Add a welcome screen hide() function, and call it from a new observer
  of the hidden attribute, when the screen becomes hidden
- Call hideAllScreens() also when leaving oobe_welcome via onBeforeHide
- Mark the active screen as not hidden in the welcome screen onBeforeShow(),
  to handle going back from the network and debug screens

R=rsorokin@google.com
Signed-off-by: Guido Trotter <ultrotter@chromium.org>
Change-Id: Id5eecb1fa88bab598ab16e5152051fc8c14a6ca9
Fixed: 1123835
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2529100Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Reviewed-by: Denis Kuznetsov [CET] <antrim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827712}