Add API to set thread name

Add API to set thread name

BUG=
R=chinmaygarde@google.com, rmacnak@google.com

Committed: https://github.com/dart-lang/sdk/commit/6302c1cc41afe773740cd8f91fed6c1b2f27007e

[Cutch, 2016-04-01 20:31:33]

Description was changed from

==========
Add API to set thread name

BUG=
==========

to

==========
Add API to set thread name

BUG=
==========

[Cutch, 2016-04-01 20:31:33]

johnmccutchan@google.com changed reviewers:
+ chinmaygarde@google.com, rmacnak@google.com

[Chinmay, 2016-04-01 21:18:53]

lgtm (not an owner). Do you want to remove |Dart_Timeline_Event_Metadata| now
that there will be no users?

[rmacnak, 2016-04-01 21:44:13]

lgtm - I don't there's any reason to keep the metadata event kind since they
deal with pids or tids that the current API doesn't allow one to set.

[Cutch, 2016-04-04 14:04:43]

On 2016/04/01 21:44:13, rmacnak wrote:
> lgtm - I don't there's any reason to keep the metadata event kind since they
> deal with pids or tids that the current API doesn't allow one to set.

I've removed Dart_Timeline_Event_Metadata and fixed a locking issue. 

Chinmay- FYI You can only call Dart_SetThreadName *after* Dart_Initialize has
been called.

[Cutch, 2016-04-04 14:13:01]

Description was changed from

==========
Add API to set thread name

BUG=
==========

to

==========
Add API to set thread name

BUG=
R=chinmaygarde@google.com, rmacnak@google.com

Committed:
https://github.com/dart-lang/sdk/commit/6302c1cc41afe773740cd8f91fed6c1b2f27007e
==========

[Cutch, 2016-04-04 14:13:02]

Committed patchset #4 (id:60001) manually as
6302c1cc41afe773740cd8f91fed6c1b2f27007e (presubmit successful).