Headless: make URLRequestDispatcher aware of navigations

DeterministicDispatcher needs to know about pending navigations in
order to ensure determinism.

headless_shell now uses this in --deterministic-fetch mode.

BUG=546953

Review-Url: https://codereview.chromium.org/2687083002
Cr-Commit-Position: refs/heads/master@{#449329}
13 files changed