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This PR changes these functions to return TaskQueueWithTaskType
- MainThreadSchedulerImpl::DefaultTaskRunner()
- MainThreadSchedulerImpl::InputTaskRunner()
- MainThreadSchedulerImpl::IPCTaskRunner()
- IdleHelper::IdleTaskRunner()

so that we can know the task types posted to these main-thread task
queues, which didin't have any task-type information.

Bug: 841172
Change-Id: I7c463437af1dd5478cbbfd39bdc1626ef2e5c3b3
Reviewed-on: https://chromium-review.googlesource.com/1063670Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559503}