Adds enableVirtualTime and setVirtualTimePolicy To Emulation domain.

This allows virtual time to be controlled via DevTools protocol.

For more details about virtual time see here: https://docs.google.com/document/d/1y9KDT_ZEzT7pBeY6uzVt1dgKlwc1OB_vY4NZO1zBQmo/edit

BUG=546953

Review-Url: https://codereview.chromium.org/2109843003
Cr-Commit-Position: refs/heads/master@{#403943}
13 files changed