Reland of Use sched_yield on all POSIX platforms in PlatformThread.

webrtc/base/platform_thread.cc

Index: webrtc/base/platform_thread.cc
diff --git a/webrtc/base/platform_thread.cc b/webrtc/base/platform_thread.cc
index 525f0ddf08d7e5959383c5c821dc36b823d4cc39..bb838a25537196faff0cb8fce89f5cdf47b37eb2 100644
--- a/webrtc/base/platform_thread.cc
+++ b/webrtc/base/platform_thread.cc
@@ -221,9 +221,6 @@
     return;
   }
 // TODO(tommi): Delete the below.
-#if !defined(WEBRTC_MAC) && !defined(WEBRTC_WIN)
-  const struct timespec ts_null = {0};
-#endif
   do {
     // The interface contract of Start/Stop is that for a successful call to
     // Start, there should be at least one call to the run function.  So we
@@ -235,11 +232,7 @@
     SleepEx(0, true);
   } while (!stop_);
 #else
-#if defined(WEBRTC_MAC)
     sched_yield();
-#else
-    nanosleep(&ts_null, nullptr);
-#endif
   } while (!AtomicOps::AcquireLoad(&stop_flag_));
 #endif  // defined(WEBRTC_WIN)
 }