Remove stale-while-revalidate experimental implementation.

This is in preparation for adding the new implementation.

This is a partial revert of commit
64c07d7959386d30a0e4aa1371e2bdcd3d436476. Parts which will still be
needed in the new implementation have been left.

BUG=348877
TEST=net_unittests

Review URL: https://codereview.chromium.org/1041763002

Cr-Commit-Position: refs/heads/master@{#335252}
14 files changed