This selects the smallest image within a srcset for data saver users.
This creates a feature flag to control this functionality.

Bug: 606941
Change-Id: I1cbb6e543035d9803bb041fb9df1e5cd8c286801
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2044810Reviewed-by: rajendrant <rajendrant@chromium.org>
Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Michael Crouse <mcrouse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742938}