//net changes for stale-while-revalidate

Add load flag LOAD_SUPPORT_ASYNC_REVALIDATION, HttpResponseInfo flag
async_revalidation_required and logic to HttpCache::Transaction to use
them.

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

BUG=348877
TEST=net_unittests

Review URL: https://codereview.chromium.org/1039263002

Cr-Commit-Position: refs/heads/master@{#345037}
6 files changed