Fix occasional race in VideoCapturerTrackSource seen by memcheck bot.

Fix occasional race in VideoCapturerTrackSource seen by memcheck bot.

The issue was that VideoCapturerTrackSource was adding a reference
to itself, causing it to not be deleted even after no external objects
reference it. The objects underneath it (threads for instance) may
then be destroyed before the object dereferences them.

BUG=webrtc:6487
Review-Url: https://codereview.webrtc.org/2717023002
Cr-Commit-Position: refs/heads/master@{#16841}
Committed: https://chromium.googlesource.com/external/webrtc/+/f15fb452efe5a7082dd49faceba1ff123985f258

[Taylor Brandstetter, 2017-02-25 11:19:09]

deadbeef@webrtc.org changed reviewers:
+ pthatcher@webrtc.org, tommi@webrtc.org

[Taylor Brandstetter, 2017-02-25 11:19:10]

Note that an additional bit of safety is that the destructor should always be
run on the signaling thread, so the async-invoked task can't be run concurrently
with the destructor. But even if that changed in the future, this should be
safe.

[tommi, 2017-02-25 14:06:58]

lgtm. good job tracking this down and improving our utilities along the way to
support these exceptional cases rather than hacking around it.

[Taylor Brandstetter, 2017-02-25 21:03:23]

The CQ bit was checked by deadbeef@webrtc.org

[Taylor Brandstetter, 2017-02-25 21:03:23]

The patchset sent to the CQ was uploaded after l-g-t-m from tommi@webrtc.org
Link to the patchset: https://codereview.webrtc.org/2717023002/#ps20001 (title:
"Rebase onto master")

[commit-bot: I haz the power, 2017-02-25 21:03:26]

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, 2017-02-25 21:37:59]

CQ is committing da patch.

Bot data: {"patchset_id": 20001, "attempt_start_ts": 1488056603214590,
"parent_rev": "ccaaffb44a7f98e68a6763675a2cc8ae02635d6e", "commit_rev":
"f15fb452efe5a7082dd49faceba1ff123985f258"}

[commit-bot: I haz the power, 2017-02-25 21:38:02]

Description was changed from

==========
Fix occasional race in VideoCapturerTrackSource seen by memcheck bot.

The issue was that VideoCapturerTrackSource was adding a reference
to itself, causing it to not be deleted even after no external objects
reference it. The objects underneath it (threads for instance) may
then be destroyed before the object dereferences them.

BUG=webrtc:6487
==========

to

==========
Fix occasional race in VideoCapturerTrackSource seen by memcheck bot.

The issue was that VideoCapturerTrackSource was adding a reference
to itself, causing it to not be deleted even after no external objects
reference it. The objects underneath it (threads for instance) may
then be destroyed before the object dereferences them.

BUG=webrtc:6487

Review-Url: https://codereview.webrtc.org/2717023002
Cr-Commit-Position: refs/heads/master@{#16841}
Committed:
https://chromium.googlesource.com/external/webrtc/+/f15fb452efe5a7082dd49face...
==========

[commit-bot: I haz the power, 2017-02-25 21:38:03]

Committed patchset #2 (id:20001) as
https://chromium.googlesource.com/external/webrtc/+/f15fb452efe5a7082dd49face...