update_engine: migrate usage of deprecated base::Value methods

Methods provided by base::Value that consume or produce unmanaged raw
pointers have been deprecated and will be eventually removed from
libchrome. This CL migrates the usage of these deprecated methods in the
update-engine code to the equivalent methods that use std::unique_ptr.

BUG=chromium:689697
TEST=Run unit tests.

Change-Id: I6a1af33735c8114fd4600618f647d5adc0a89603
Reviewed-on: https://chromium-review.googlesource.com/438840


Commit-Ready: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>