Expose platform thread id on blink::WebThread

Expose platform thread id on blink::WebThread

DevTools code needs access to platform thread id in order to collect Worker thread ids and based on them differentiate worker threads in recorded trace events.

Blink part of the change: https://codereview.chromium.org/514193002/

BUG=401895
Committed: https://crrev.com/07e7a118841a8f06d6634d20b978b5a3bcf183ac
Cr-Commit-Position: refs/heads/master@{#294345}

[yurys, 2014-08-28 15:30:14]

yurys@chromium.org changed reviewers:
+ darin@chromium.org

[yurys, 2014-09-09 09:07:59]

@darin: can you take a look please?

[yurys, 2014-09-10 06:13:21]

On 2014/09/09 09:07:59, yurys wrote:
> @darin: can you take a look please?

@darin: it seems that you reviewed the Blink part of the change (referenced in
the description of this CL), but I actually need a review for the Chromium side
change (this CL). PTAL

[yurys, 2014-09-11 05:40:01]

yurys@chromium.org changed reviewers:
+ jamesr@chromium.org, jochen@chromium.org

[yurys, 2014-09-11 05:40:02]

@jamesr: please review mojo/ changes
@jochen: please review content/ changes

[darin (slow to review), 2014-09-11 06:08:14]

LGTM, sorry about that!

[yurys, 2014-09-11 06:14:32]

On 2014/09/11 06:08:14, darin wrote:
> LGTM, sorry about that!

np, thank you!

[yurys, 2014-09-11 06:14:37]

The CQ bit was checked by yurys@chromium.org

[commit-bot: I haz the power, 2014-09-11 06:24:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/515243002/20001

[commit-bot: I haz the power, 2014-09-11 07:29:01]

Committed patchset #2 (id:20001) as da6986dafdf87a98d0eb024dde681be7746f3724

[commit-bot: I haz the power, 2014-09-11 07:38:27]

Patchset 2 (id:??) landed as
https://crrev.com/07e7a118841a8f06d6634d20b978b5a3bcf183ac
Cr-Commit-Position: refs/heads/master@{#294345}