content::ResourceDispatcherHostImpl changes for stale-while-revalidate

Inform //net that we support async revalidation using the net::LOAD_SUPPORT_ASYNC_REVALIDATION flag. Issue async revalidations when response info had the async_revalidation_required flag.

See design doc at
https://docs.google.com/a/chromium.org/document/d/1nBhr25nSJgoyAh4S1-U5h2sH70Iz4RR0NAfXNL79G5Y/edit

BUG=348877
TEST=content_unittests

Review URL: https://codereview.chromium.org/1041993004

Cr-Commit-Position: refs/heads/master@{#364253}
16 files changed