This cl refactor TaskQueues to use a PIMPL implementation on linux/Android.

In later steps the Win/Mac implementation will also be refactored.

The rtc_task_queue target is split up in three separate targets:

rtc_task_queue_api:
Contains the header file task_queue.h but no implementation.
Only external TaskQueue implementations should directly depend on this target.

rtc_task_queue_impl:
Contains the default implementation of task_queue.h.
Only external application targets should directly depend on this target.

rtc_task_queue:
WebRTC targets should depend on this target. It unconditionally depend on rtc_task_queue_api and depending on the new build flag,|rtc_link_task_queue_impl|,  depend on rtc_task_queue_impl.

BUG=webrtc:8160

Review-Url: https://codereview.webrtc.org/3003643002
Cr-Original-Commit-Position: refs/heads/master@{#19516}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 650fdae91c690fb9904f72873daeb81415ff9ba1
4 files changed