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 785287fcf6189a202115c168a5a085508a0f4c9b..6dfc5c551db0b2e2dff034716ab0188c95062176 100644
--- a/base/message_loop/message_loop.cc
+++ b/base/message_loop/message_loop.cc
@@ -171,6 +171,7 @@ MessageLoop::~MessageLoop() {
   // Tell the incoming queue that we are dying.
   incoming_task_queue_->WillDestroyCurrentMessageLoop();
   incoming_task_queue_ = NULL;
+  unbound_task_runner_ = NULL;
   task_runner_ = NULL;
 
   // OK, now make it so that no one can find us.
@@ -368,6 +369,7 @@ bool MessageLoop::IsIdleForTesting() {
 
 //------------------------------------------------------------------------------
 
+// static
 scoped_ptr<MessageLoop> MessageLoop::CreateUnbound(
     Type type, MessagePumpFactoryCallback pump_factory) {
   return make_scoped_ptr(new MessageLoop(type, pump_factory));
@@ -387,7 +389,9 @@ MessageLoop::MessageLoop(Type type, MessagePumpFactoryCallback pump_factory)
       message_histogram_(NULL),
       run_loop_(NULL),
       incoming_task_queue_(new internal::IncomingTaskQueue(this)),
-      task_runner_(new internal::MessageLoopTaskRunner(incoming_task_queue_)) {
+      unbound_task_runner_(
+          new internal::MessageLoopTaskRunner(incoming_task_queue_)),
+      task_runner_(unbound_task_runner_) {
   // If type is TYPE_CUSTOM non-null pump_factory must be given.
   DCHECK_EQ(type_ == TYPE_CUSTOM, !pump_factory_.is_null());
 }
@@ -403,7 +407,25 @@ void MessageLoop::BindToCurrentThread() {
   lazy_tls_ptr.Pointer()->Set(this);
 
   incoming_task_queue_->StartScheduling();
-  task_runner_->BindToCurrentThread();
+  unbound_task_runner_->BindToCurrentThread();
+  unbound_task_runner_ = nullptr;
+  SetThreadTaskRunnerHandle();
+}
+
+void MessageLoop::SetTaskRunner(
+    scoped_refptr<SingleThreadTaskRunner> task_runner) {
+  DCHECK_EQ(this, current());
+  DCHECK(task_runner->BelongsToCurrentThread());
+  DCHECK(!unbound_task_runner_);
+  task_runner_ = task_runner.Pass();
+  SetThreadTaskRunnerHandle();
+}
+
+void MessageLoop::SetThreadTaskRunnerHandle() {
+  DCHECK_EQ(this, current());
+  // Clear the previous thread task runner first because only one can exist at
+  // a time.
+  thread_task_runner_handle_.reset();
   thread_task_runner_handle_.reset(new ThreadTaskRunnerHandle(task_runner_));
 }