Add gn target rtc_task_runner.

Add gn target rtc_task_runner.

This is step 1 in the following process to move the task runner
abstraction over to Chrome, without gettings link errors on duplicate
symbols.

1. Move files from the rtc_base target to a new target
   rtc_task_runner, and let rtc_base publicly depend on it.

2. In Chrome, add an explicit dependency on rtc_task_runner where it
   depends on rtc_base.

3. Drop the webrtc dependency rtc_base --> rtc_task_runner.

4. Copy task runner code to Chrome (cl
   https://codereview.chromium.org/2694903005/), and drop its
   dependency on webrtc's rtc_task_runner target.

5. Delete the rtc_task_runner target and corresponding source files
   from webrtc. Mission accomplished!

BUG=webrtc:6424
Review-Url: https://codereview.webrtc.org/2696703009
Cr-Commit-Position: refs/heads/master@{#16710}
Committed: https://chromium.googlesource.com/external/webrtc/+/915bbd53e477d86515b9782d66b3a7275a172fa5

[nisse-webrtc, 2017-02-16 13:32:17]

nisse@webrtc.org changed reviewers:
+ kjellander@webrtc.org, pthatcher@webrtc.org

[nisse-webrtc, 2017-02-16 13:32:18]

I can't convince gn/ninja to produce a working link library order in the
presence of multiple implementations of taskrunner and related classes. So let's
try this detour.

[kjellander_webrtc, 2017-02-16 14:37:16]

lgtm, thanks for taking the time to do this!

[pthatcher1, 2017-02-17 18:20:49]

lgtm

[nisse-webrtc, 2017-02-20 07:40:13]

The CQ bit was checked by nisse@webrtc.org

[commit-bot: I haz the power, 2017-02-20 07:40:16]

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, 2017-02-20 08:50:23]

CQ is committing da patch.

Bot data: {"patchset_id": 1, "attempt_start_ts": 1487576413158300, "parent_rev":
"bf25bbdc6380e1c629863f24457289c6aa9813ea", "commit_rev":
"915bbd53e477d86515b9782d66b3a7275a172fa5"}

[commit-bot: I haz the power, 2017-02-20 08:50:25]

Description was changed from

==========
Add gn target rtc_task_runner.

This is step 1 in the following process to move the task runner
abstraction over to Chrome, without gettings link errors on duplicate
symbols.

1. Move files from the rtc_base target to a new target
   rtc_task_runner, and let rtc_base publicly depend on it.

2. In Chrome, add an explicit dependency on rtc_task_runner where it
   depends on rtc_base.

3. Drop the webrtc dependency rtc_base --> rtc_task_runner.

4. Copy task runner code to Chrome (cl
   https://codereview.chromium.org/2694903005/), and drop its
   dependency on webrtc's rtc_task_runner target.

5. Delete the rtc_task_runner target and corresponding source files
   from webrtc. Mission accomplished!

BUG=webrtc:6424
==========

to

==========
Add gn target rtc_task_runner.

This is step 1 in the following process to move the task runner
abstraction over to Chrome, without gettings link errors on duplicate
symbols.

1. Move files from the rtc_base target to a new target
   rtc_task_runner, and let rtc_base publicly depend on it.

2. In Chrome, add an explicit dependency on rtc_task_runner where it
   depends on rtc_base.

3. Drop the webrtc dependency rtc_base --> rtc_task_runner.

4. Copy task runner code to Chrome (cl
   https://codereview.chromium.org/2694903005/), and drop its
   dependency on webrtc's rtc_task_runner target.

5. Delete the rtc_task_runner target and corresponding source files
   from webrtc. Mission accomplished!

BUG=webrtc:6424

Review-Url: https://codereview.webrtc.org/2696703009
Cr-Commit-Position: refs/heads/master@{#16710}
Committed:
https://chromium.googlesource.com/external/webrtc/+/915bbd53e477d86515b9782d6...
==========

[commit-bot: I haz the power, 2017-02-20 08:50:26]

Committed patchset #1 (id:1) as
https://chromium.googlesource.com/external/webrtc/+/915bbd53e477d86515b9782d6...