Add counterfactual arm for clipboard image searches

The image option is unlikely to appear very often. This CL adds a
counterfactual arm to the feature so we can get a better comparison
with what would be shown if the feature were enabled.

Bug: 913958
Change-Id: I02a0e39415e25a64aa7a2f22602b632e1b38776e
Reviewed-on: https://chromium-review.googlesource.com/c/1477240
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634157}
2 files changed