Prepare for per-webview virtual time

This patch removes the initial global virtual time implementation (including the flag)
and prepares for per-webview virtual time.  A follow on patch will wire this
up in blink: https://codereview.chromium.org/1646583002/

BUG=546953

Review URL: https://codereview.chromium.org/1652083002

Cr-Commit-Position: refs/heads/master@{#373032}
41 files changed