Add ability to control StaleWhileRevalidate via OriginTrials

The implementation isn't there yet but this flag controls whether
the eventual implementation should do something or not.

BUG=348877

Change-Id: I72a5a0eb2873d0a06ae5ce84c494b4d24b91434d
Reviewed-on: https://chromium-review.googlesource.com/1100942
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567974}
6 files changed