Convert mac/ios TaskQueue implementation to pimpl convention.

Convert mac/ios TaskQueue implementation to pimpl convention.

BUG=webrtc:8160, webrtc:8166
Review-Url: https://codereview.webrtc.org/3013433002
Cr-Commit-Position: refs/heads/master@{#19743}
Committed: https://chromium.googlesource.com/external/webrtc/+/66d07c54fef7b0ddd5418f05096757e2acc8308e

[nisse-webrtc, 2017-09-06 14:24:50]

nisse@webrtc.org changed reviewers:
+ denicija@webrtc.org, tommi@webrtc.org

[nisse-webrtc, 2017-09-06 14:24:52]

PTAL. Like for the recent windows cl (https://codereview.webrtc.org/3009133002),
I haven't really tried to understand how the code works, just moved it into
TaskQueue::Impl.

It was made a little different from the other implementations, because this uses
AutoSetCurrentQueuePtr, and that means that the tls pointer needs to point to a
TaskQueue, rather than a TaskQueue::Impl or some other context struct. If that's
better or worse, I don't know, but it makes the pimpl glue code slightly
different than for the windows and libevent code..

[tommi, 2017-09-08 08:32:10]

lgtm assuming all tests pass (I'm not sure the mac_rel bot runs everything)

[daniela-webrtc, 2017-09-08 09:22:24]

lgtm

[nisse-webrtc, 2017-09-08 11:39:41]

The CQ bit was checked by nisse@webrtc.org

[commit-bot: I haz the power, 2017-09-08 11:39:51]

CQ is trying da patch.

Follow status at:
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.webrtc.org/...

[commit-bot: I haz the power, 2017-09-08 12:12:52]

CQ is committing da patch.

Bot data: {"patchset_id": 100001, "attempt_start_ts": 1504870781368910,
"parent_rev": "a64685325c2f8f51873b67ae8a91f94ffb19d70b", "commit_rev":
"66d07c54fef7b0ddd5418f05096757e2acc8308e"}

[commit-bot: I haz the power, 2017-09-08 12:12:56]

Description was changed from

==========
Convert mac/ios TaskQueue implementation to pimpl convention.

BUG=webrtc:8160, webrtc:8166
==========

to

==========
Convert mac/ios TaskQueue implementation to pimpl convention.

BUG=webrtc:8160, webrtc:8166

Review-Url: https://codereview.webrtc.org/3013433002
Cr-Commit-Position: refs/heads/master@{#19743}
Committed:
https://chromium.googlesource.com/external/webrtc/+/66d07c54fef7b0ddd5418f050...
==========

[commit-bot: I haz the power, 2017-09-08 12:12:58]

Committed patchset #6 (id:100001) as
https://chromium.googlesource.com/external/webrtc/+/66d07c54fef7b0ddd5418f050...