base: Make it possible to replace the MessageLoop's task runner

base/message_loop/message_loop.cc

Index: base/message_loop/message_loop.cc
diff --git a/base/message_loop/message_loop.cc b/base/message_loop/message_loop.cc
index 74bfdb6f4ef6c2d16f66891c8189f8a5ffb76613..219d520f6d9e52e1bd50983d86f17ea62ee628b1 100644
--- a/base/message_loop/message_loop.cc
+++ b/base/message_loop/message_loop.cc
@@ -367,6 +367,7 @@ bool MessageLoop::IsIdleForTesting() {
 
 //------------------------------------------------------------------------------
 
+// static
 scoped_ptr<MessageLoop> MessageLoop::CreateUnbound(
     Type type, MessagePumpFactoryCallback pump_factory) {
   return make_scoped_ptr(new MessageLoop(type, pump_factory));
@@ -403,7 +404,25 @@ void MessageLoop::BindToCurrentThread() {
 
   incoming_task_queue_->StartScheduling();
   task_runner_->BindToCurrentThread();
-  thread_task_runner_handle_.reset(new ThreadTaskRunnerHandle(task_runner_));
+  scoped_refptr<BindableSingleThreadTaskRunner> previous_task_runner =
+      SwapTaskRunner(task_runner_.Pass());
+  DCHECK(previous_task_runner == NULL);
+}
+
+scoped_refptr<BindableSingleThreadTaskRunner> MessageLoop::SwapTaskRunner(
+    scoped_refptr<BindableSingleThreadTaskRunner> task_runner) {
+  // If this message loop was already bound to a thread, ensure the task runner
+  // belongs to the same thread and update the thread task runner handle.
+  // Otherwise the task runner will be bound later in BindToCurrentThread().
+  if (pump_) {
+    DCHECK_EQ(this, current());
+    DCHECK(task_runner->BelongsToCurrentThread());
+    // This reset() is needed or ThreadTaskRunnerHandle::Get() will crash.
+    thread_task_runner_handle_.reset();
+    thread_task_runner_handle_.reset(new ThreadTaskRunnerHandle(task_runner));
+  }
+  task_runner_.swap(task_runner);
+  return task_runner;
 }
 
 void MessageLoop::RunHandler() {