Force non-joinable SimpleThreads to be leaked.

base/threading/simple_thread.h

Index: base/threading/simple_thread.h
diff --git a/base/threading/simple_thread.h b/base/threading/simple_thread.h
index dd1fc6c18e6d4f250e2757e1e0adbabade1ec6ff..f06a3169c2e7e193e10b95216b6c515367634867 100644
--- a/base/threading/simple_thread.h
+++ b/base/threading/simple_thread.h
@@ -74,9 +74,14 @@ class BASE_EXPORT SimpleThread : public PlatformThread::Delegate {
 
     ThreadPriority priority = ThreadPriority::NORMAL;
 
-    // If false, the thread's PlatformThreadHandle will not be kept around and
-    // the SimpleThread instance will not be required to be Join()'ed before
-    // being destroyed
+    // If false, the underlying thread's PlatformThreadHandle will not be kept
+    // around and as such the SimpleThread instance will not be Join()able and
+    // will have to be leaked. Useful for threads that want to have
+    // TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN semantics. Note: leaking a
+    // joinable thread would effictively have the same semantics, but making it
+    // non-joinable is cleaner as not keeping the PlatformThreadHandle around
+    // has the advantage of letting the system release resources associated with
+    // the thread immediately when it returns from ThreadMain().
     bool joinable = true;
   };