Expose RunsTasksOnCurrentThread through WebTaskRunner

RunsTasksOnCurrentThread will be used by assertions in tasks that checks
if it runs on the correct thread. WebTaskRunner itself will use it in
the following CL.

Blink code starts using per-frame WebTaskRunner rather than global
WebThread-associated TaskRunner. RunTasksOnCurrentThread is a missing
functionality to migrate from thread-based task scheduling to TaskRunner
based scheduling.

Review-Url: https://codereview.chromium.org/2123783002
Cr-Commit-Position: refs/heads/master@{#406537}
10 files changed