Add StaleRevalidationResourceClient fetcher.

When a stale resource is detected a subsequent request will be queued
via a StaleRevalidateResourceClient. When the subsequent resource
fetch completes it will be removed from the MemoryCache.

Design: https://docs.google.com/document/d/1wqbhCdf3eCv-VuUoV34xD2KfMinOCxH4fRe5EDCtXIo/edit

BUG=348877

Change-Id: I27ba8f295fe070e7a34eed7a362921ab4ce544cf
Reviewed-on: https://chromium-review.googlesource.com/1103064
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572606}
15 files changed