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When initializing demo session (or preloading demo resoruces), register
and attempt to load demo mode resources component.

If demo mode resources component is not installed, demo_session will
attempt to load demo resources from a preinstalled path on the stateful
partition (if that fails, the session will continue without offline
demo mode resources).

To avoid CrOSComponentManager::Load blocking on component update if the
component is not installed, add new UpdatePolicy - kSkip. If set,
CrOSComponentManager will report error if a compatible component version
is not found after component registration. CrOSComponentManager::Load
will srill register the component with component updated, which means that
the component will eventually get updated, but the updater will not be run
as part of the requested component load.

NOTE: Currently, demo-mode-resources component load is expected to fail,
as the component config has still not been added to the
cros_component_installer_chromeos (this is blocked on server changes).

BUG=869455

Change-Id: I2c3b18d60edc44ae8ddbde27d6f31e91092216dd
Reviewed-on: https://chromium-review.googlesource.com/1164577
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582790}