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Instead of downloading container at browser start, we now have a dbus
API to download container at any time.

BUG=chromium:768009
TEST=manual

Change-Id: I00f57024b27e4dc8ad2ba44f7ed7cbb5e374afbc
Reviewed-on: https://chromium-review.googlesource.com/688055
Commit-Queue: Xiaochu Liu <xiaochu@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504802}