Consider PlatformThread::Join() to be blocking IO.

base/platform_thread_win.cc

Index: base/platform_thread_win.cc
diff --git a/base/platform_thread_win.cc b/base/platform_thread_win.cc
index e5afc52e6ff0d51e8cdbe4fdc77a92e9865d0f82..68e837e7a0e826f9cd284fd50573c91ad35fe0a8 100644
--- a/base/platform_thread_win.cc
+++ b/base/platform_thread_win.cc
@@ -125,6 +125,10 @@ bool PlatformThread::CreateNonJoinable(size_t stack_size, Delegate* delegate) {
 // static
 void PlatformThread::Join(PlatformThreadHandle thread_handle) {
   DCHECK(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.
+  base::ThreadRestrictions::AssertIOAllowed();
 
   // Wait for the thread to exit.  It should already have terminated but make
   // sure this assumption is valid.