Always use TaskScheduler::InitParams to initialize a TaskScheduler.

This CL removes code that allows initialization of TaskSchedulerImp
with a vector of SchedulerWorkerPoolParams and a TaskTraits->pool
mapping function. TaskSchedulerImpl is now always initialized with a
TaskScheduler::InitParams, always has four pools and has a hard-coded
TaskTraits->pool mapping function.

BUG=690706

Review-Url: https://codereview.chromium.org/2809163003
Cr-Commit-Position: refs/heads/master@{#465583}
9 files changed