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

base/platform_thread_posix.cc

Index: base/platform_thread_posix.cc
diff --git a/base/platform_thread_posix.cc b/base/platform_thread_posix.cc
index e442e9c4571b38859597a68cf1ed3a376271b222..9807ac60ec2b78c0a5d74cb7727c99aa20b79dd9 100644
--- a/base/platform_thread_posix.cc
+++ b/base/platform_thread_posix.cc
@@ -219,5 +219,9 @@ bool PlatformThread::CreateNonJoinable(size_t stack_size, Delegate* delegate) {
 
 // static
 void PlatformThread::Join(PlatformThreadHandle 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();
   pthread_join(thread_handle, NULL);
 }