Remove thread id from ThreadWrapper::Start().

Removes ThreadPosix::InitParams and a corresponding wait for an event.
This unblocks ThreadPosix::Start which had to wait for thread scheduling
for an event to trigger on the spawned thread, giving faster Start()
calls.

BUG=4413
R=tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/43699004

Cr-Commit-Position: refs/heads/master@{#8709}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8709 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/audio_device/ios/audio_device_ios.h

@@ -215,7 +215,6 @@ class AudioDeviceIOS : public AudioDeviceGeneric {
   CriticalSectionWrapper& _critSect;
 
   ThreadWrapper* _captureWorkerThread;
-  uint32_t _captureWorkerThreadId;
 
   int32_t _id;