Use DCHECK_EQ() when comparing TimeDeltas.

This CL is a part of https://codereview.chromium.org/665023002/ ("Post-commit
fixes for "stale-while-revalidate..."") that has been separated into a separate
CL because it depends on https://codereview.chromium.org/669083002/ ("Add
logging support for base::Time* types.").

BUG=348877

Review URL: https://codereview.chromium.org/670303003

Cr-Commit-Position: refs/heads/master@{#302065}
1 file changed