RTCVideoDecoder: add Destroy() static method for RTCVideoDecoderFactory

RTCVideoDecoder: add Destroy() static method for RTCVideoDecoderFactory

RTCVDFactory does not know where RTCVideoDecoder is created (it is
created in one of the parameter's task runner), however it knows
where to destroy it. Address this imbalance by offering an
RTCVideoDecoder::Destroy() where the threading is explicit, since
it lives next to RTCVideoDecoder::Create().

BUG=554737
TEST= tiny change, all unit tests passing.

Committed: https://crrev.com/9fa42c24df5eb91352e1f1a7f0989007f3024b74
Cr-Commit-Position: refs/heads/master@{#360606}

[mcasas, 2015-11-18 02:42:32]

mcasas@chromium.org changed reviewers:
+ posciak@chromium.org

[mcasas, 2015-11-18 02:42:33]

posciak@ PTAL (straightforward code shifting).

[Pawel Osciak, 2015-11-19 08:59:54]

lgtm

[mcasas, 2015-11-19 15:49:20]

The CQ bit was checked by mcasas@chromium.org

[commit-bot: I haz the power, 2015-11-19 15:50:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1458683002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1458683002/1

[commit-bot: I haz the power, 2015-11-19 16:49:40]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-19 16:50:22]

Patchset 1 (id:??) landed as
https://crrev.com/9fa42c24df5eb91352e1f1a7f0989007f3024b74
Cr-Commit-Position: refs/heads/master@{#360606}