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 9dd16e39e3e53f88c961b0e64d4d3ee183f713e6..9a6a2bb999c4fe80adc83f874a0a08ea7b72116c 100644
--- a/base/threading/platform_thread_posix.cc
+++ b/base/threading/platform_thread_posix.cc
@@ -16,6 +16,7 @@
 
 #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"
@@ -210,6 +211,9 @@ bool PlatformThread::CreateNonJoinableWithPriority(size_t stack_size,
 
 // 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.