Add a WorkerScheduler and a WebThreadImplForWorker
Adds a simple WorkerScheduler which supports Default and Idle Tasks (via
Long Idle Periods). A follow up Blink patch will refactor
WorkerThread.cpp to use these Idle Tasks.
A WorkerSceduler is a pre-requisite for refactoring away the Blink
Timer Heap as described in:
https://docs.google.com/document/d/163ow-1wjd6L0rAN3V_U6t12eqVkq4mXDDjVaA4OuvCA/edit?usp=sharing
BUG=463143
Review URL: https://codereview.chromium.org/
1033643004
Cr-Commit-Position: refs/heads/master@{#324847}