Enable ChildThreadImpl to refer ChildScheduler for IPC task runner

Now ChildThreadImpl needs to pass an IPC listener task runner to create
IPC::SyncChannel, but ChildThreadImpl can't refer schedulers. This CL
enables ChildThreadImpl to refer ChildScheduler by adding ChildScheduler
::IPCListenerTaskRunner function. Now the implementation
(RendererScheduler::IPCListenerTaskRunner) just returns base::
ThreadTaskRunnerHandle::Get().

This CL is just a preparation for flexible scheduling, and doesn't
change any behavior so far.

Bug: 780785
Change-Id: I46965cab95ca6f98dee56299df9ebea1462886d8
Reviewed-on: https://chromium-review.googlesource.com/760336
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515913}

• content/child/child_thread_impl.cc[diff]
• content/child/child_thread_impl.h[diff]
• content/renderer/render_thread_impl.cc[diff]
• third_party/WebKit/Source/platform/scheduler/child/compositor_worker_scheduler.cc[diff]
• third_party/WebKit/Source/platform/scheduler/child/compositor_worker_scheduler.h[diff]
• third_party/WebKit/Source/platform/scheduler/child/worker_scheduler_impl.cc[diff]
• third_party/WebKit/Source/platform/scheduler/child/worker_scheduler_impl.h[diff]
• third_party/WebKit/Source/platform/scheduler/renderer/renderer_scheduler_impl.cc[diff]
• third_party/WebKit/Source/platform/scheduler/renderer/renderer_scheduler_impl.h[diff]
• third_party/WebKit/Source/platform/scheduler/test/fake_renderer_scheduler.cc[diff]
• third_party/WebKit/public/platform/scheduler/child/child_scheduler.h[diff]
• third_party/WebKit/public/platform/scheduler/test/fake_renderer_scheduler.h[diff]
• third_party/WebKit/public/platform/scheduler/test/mock_renderer_scheduler.h[diff]