Add an about:flag to support alternative data saver features
This flag allows an alternative back end Data Saver implementation to be
used by chrome. This may result in different types of previews served,
different triggering mechanisms, or other back end changes that are not
used by default.
This also fixes the CP header having multiple "exp" directives.
BUG=721113,656195
Review-Url: https://codereview.chromium.org/2887423002
Cr-Commit-Position: refs/heads/master@{#474320}
17 files changed
