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This CL initializes TaskScheduler in renderers using
default arguments. ContentRenderClient may provide
different arguments.

Initially, no threads will be created by TaskScheduler
in renderers because no tasks are posted to it and
pools are initialized with StandbyThreadPolicy::LAZY.

In a separate, we will start running v8 tasks in
TaskScheduler rather than WorkerPool
(https://cs.chromium.org/chromium/src/gin/v8_platform.cc?sq=package:chromium&l=70&dr=C).

BUG=664996

Review-Url: https://codereview.chromium.org/2491823005
Cr-Commit-Position: refs/heads/master@{#438202}