arc: Add file-picker-experiment feature flag

and fill StartArcMiniContainerRequest with the flag value in arc_session_impl

Refrence CL: http://crrev.com/c/617748
TEST=Manually confirmed that the flag value is passed to arc-setup
BUG=b:117814025

Change-Id: If2eb9a71fcd565af45aba7de77cc40f80acb7f37
Reviewed-on: https://chromium-review.googlesource.com/c/1295974
Commit-Queue: Satoshi Niwa <[email protected]>
Reviewed-by: Hidehiko Abe <[email protected]>
Reviewed-by: Yusuke Sato <[email protected]>
Cr-Commit-Position: refs/heads/master@{#602253}
7 files changed