Allow immediate tasks to run even when virtual time can't advance

The main point of preventing virtual time from advancing is to save
CPU by avoiding running timers when we don't need to.  We still
however need immediate tasks to run so that the page load can
progress.

BUG=546953

Review URL: https://codereview.chromium.org/1697523002

Cr-Commit-Position: refs/heads/master@{#375166}
5 files changed