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Lots of TODOs, but there's enough implemented so that running "chrome
--enable-features=AppService" will populate the launcher with extension
apps' names and icons, and clicking on the icon will launch the app.

BUG=826982

Change-Id: Id424abac859dde69ffdac3c28a473fce61fbe8df
Reviewed-on: https://chromium-review.googlesource.com/c/1351975
Commit-Queue: Nigel Tao <nigeltao@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613334}