Track thread activities in order to diagnose hangs.

base/threading/platform_thread_posix.cc

Index: base/threading/platform_thread_posix.cc
diff --git a/base/threading/platform_thread_posix.cc b/base/threading/platform_thread_posix.cc
index 2321b3cd49df64bbb352bc4cfd20994db691a099..1d574f9da01a7f3ec962199d16fa7e53b7b9a5b6 100644
--- a/base/threading/platform_thread_posix.cc
+++ b/base/threading/platform_thread_posix.cc
@@ -14,6 +14,7 @@
 
 #include <memory>
 
+#include "base/debug/activity_tracker.h"
 #include "base/lazy_instance.h"
 #include "base/logging.h"
 #include "base/threading/platform_thread_internal_posix.h"
@@ -202,6 +203,9 @@ bool PlatformThread::CreateNonJoinable(size_t stack_size, Delegate* delegate) {
 
 // static
 void PlatformThread::Join(PlatformThreadHandle thread_handle) {
+  // Record the event that this thread is blocking upon (for hang diagnosis).
+  base::debug::ScopedThreadJoinActivity thread_activity(&thread_handle);
+
   // Joining another thread may block the current thread for a long time, since
   // the thread referred to by |thread_handle| may still be running long-lived /
   // blocking tasks.