Emulation.setVirtualTimePolicy to have an optional virtual time budge

If virtualTimeBudgetSeconds is set then after the specified virtual
seconds have elapsed, virtual time pauses and an
Emulation.virtualTimeBudgetExpired event is sent.

BUG=546953

Review-Url: https://codereview.chromium.org/2185033002
Cr-Commit-Position: refs/heads/master@{#409218}
4 files changed