Add parsing of stale-while-revalidate cache control field.

Add the ability to fetch the stale while revalidate field out of the
cache control header.

BUG=348877

Change-Id: I7e6132b71c10c462019687a150e259e02fe9c332
Reviewed-on: https://chromium-review.googlesource.com/1101242
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568002}
5 files changed