Disable MessageLoop features on some BrowserThreads.

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 ef7747ef06998475ede2369c110c8efcb2ba712a..23fd0751c68bf2c43ea435050e2a6f94d57fed9a 100644
--- a/base/message_loop/message_loop.cc
+++ b/base/message_loop/message_loop.cc
@@ -124,8 +124,10 @@ MessageLoop::~MessageLoop() {
   DCHECK(!did_work);
 
   // Let interested parties have one last shot at accessing this.
-  FOR_EACH_OBSERVER(DestructionObserver, destruction_observers_,
-                    WillDestroyCurrentMessageLoop());
+  if (enable_destruction_observers_) {
+    FOR_EACH_OBSERVER(DestructionObserver, destruction_observers_,
+                      WillDestroyCurrentMessageLoop());
+  }
 
   thread_task_runner_handle_.reset();
 
@@ -205,22 +207,26 @@ std::unique_ptr<MessagePump> MessageLoop::CreateMessagePumpForType(Type type) {
 void MessageLoop::AddDestructionObserver(
     DestructionObserver* destruction_observer) {
   DCHECK_EQ(this, current());
+  //DCHECK(enable_destruction_observers_);
   destruction_observers_.AddObserver(destruction_observer);
 }
 
 void MessageLoop::RemoveDestructionObserver(
     DestructionObserver* destruction_observer) {
   DCHECK_EQ(this, current());
+  //DCHECK(enable_destruction_observers_);
   destruction_observers_.RemoveObserver(destruction_observer);
 }
 
 void MessageLoop::AddNestingObserver(NestingObserver* observer) {
   DCHECK_EQ(this, current());
+  DCHECK(enable_nesting_);
   nesting_observers_.AddObserver(observer);
 }
 
 void MessageLoop::RemoveNestingObserver(NestingObserver* observer) {
   DCHECK_EQ(this, current());
+  DCHECK(enable_nesting_);
   nesting_observers_.RemoveObserver(observer);
 }
 
@@ -274,11 +280,13 @@ bool MessageLoop::IsNested() {
 
 void MessageLoop::AddTaskObserver(TaskObserver* task_observer) {
   DCHECK_EQ(this, current());
+  DCHECK(enable_task_observers_);
   task_observers_.AddObserver(task_observer);
 }
 
 void MessageLoop::RemoveTaskObserver(TaskObserver* task_observer) {
   DCHECK_EQ(this, current());
+  DCHECK(enable_task_observers_);
   task_observers_.RemoveObserver(task_observer);
 }
 
@@ -367,6 +375,7 @@ void MessageLoop::SetThreadTaskRunnerHandle() {
 
 void MessageLoop::RunHandler() {
   DCHECK_EQ(this, current());
+  DCHECK(enable_nesting_ || run_loop_->run_depth_ == 1);
   pump_->Run(this);
 }