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Adds new hotkey for ChromeVox users to quickly get
battery status. This change involves:

1. Plumbing hotkey request from ChromeVox extension
to ash, where battery status can be retrieved.

2. Setting up callbacks from ash to Browser and
and from Browser to ChromeVox so that battery status
can be reported to the user.

Change-Id: I9c2b96a49a54d152bffc1e373eec352d0a902403
Reviewed-on: https://chromium-review.googlesource.com/c/1388176
Commit-Queue: Akihiro Ota <akihiroota@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Reviewed-by: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622162}