Add |joinable| to Thread::Options

base/threading/thread.h

Index: base/threading/thread.h
diff --git a/base/threading/thread.h b/base/threading/thread.h
index 45064240d76f8436ceb7ab51ff205be4646a8485..48cf37f3f42583a75ad7e850970cec746a63697e 100644
--- a/base/threading/thread.h
+++ b/base/threading/thread.h
@@ -74,6 +74,12 @@ class BASE_EXPORT Thread : PlatformThread::Delegate {
 
     // Specifies the initial thread priority.
     ThreadPriority priority = ThreadPriority::NORMAL;
+
+    // If false, the thread will not be joined on destruction. This is intended
+    // for threads that want TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN
+    // semantics. Stop() will not be synchronous and will instead merely have
+    // StopSoon() semantics on such threads.
+    bool joinable = true;
   };
 
   // Constructor.
@@ -131,12 +137,14 @@ class BASE_EXPORT Thread : PlatformThread::Delegate {
   // carefully for production code.
   bool WaitUntilThreadStarted() const;
 
-  // Signals the thread to exit and returns once the thread has exited.  After
-  // this method returns, the Thread object is completely reset and may be used
-  // as if it were newly constructed (i.e., Start may be called again).
+  // Signals the thread to exit and returns once the thread has exited (or right
+  // away if the thread is non-joinable).  For joinable threads only: after this
+  // method returns, the Thread object is completely reset and may be used as if
+  // it were newly constructed (i.e., Start may be called again) -- non-joinable
+  // threads are not re-usable.
   //
   // Stop may be called multiple times and is simply ignored if the thread is
-  // already stopped.
+  // already stopped or currently stopping.
   //
   // NOTE: If you are a consumer of Thread, it is not necessary to call this
   // before deleting your Thread objects, as the destructor will do it.