Remove stale-while-revalidate from content and chrome

This removes the code for the stale-while-revalidate experiment from
content and the flags entry from chrome. This simplifies some c/b/loader
separation particularly in resource_dispatcher_host_impl.cc.

See https://bugs.chromium.org/p/chromium/issues/detail?id=348877#c66 for
details on the experiment turndown.

BUG=348877,700568

Review-Url: https://codereview.chromium.org/2764683002
Cr-Commit-Position: refs/heads/master@{#458613}
20 files changed