scheduler: Add an unthrottled per-frame task runner

Add an unthrottled task runner which will be used for executing internal
browser tasks which should never be throttled. In general only tasks
whose performance characteristics are known should be posted to this
task runner; for example user JavaScript should not be allowed.

BUG=624699

Review-Url: https://codereview.chromium.org/2118783002
Cr-Commit-Position: refs/heads/master@{#403670}
11 files changed