[Ephemeral Tab] Add to fieldtrial_testing_config.

Enables the Ephemeral Tab, aka "Sneak peek" context menu item, to our
testing configuration.

BUG=894619

Change-Id: I9ed3499fa74cc8124641524e79b6a0976ba7eb6b
Reviewed-on: https://chromium-review.googlesource.com/c/1368586
Reviewed-by: Jinsuk Kim <jinsukkim@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Donn Denman <donnd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636052}
2 files changed