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1) |getLaunchableApps| should only return launchable apps, but not all
   of the apps would necessarily be ready. (Non-ready apps becomes
   ready normally after ~10s after user session starts.)

2) |launchApp| can be used to launch non-ready apps. A spinner will be
   shown on the shelf and the app will launch after it becomes ready.

Bug: 819404
Change-Id: I6569043226f38b6905687defb4be95554a5e5511
Reviewed-on: https://chromium-review.googlesource.com/1141349
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Reviewed-by: Yury Khmel <khmel@chromium.org>
Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578509}