Remove DebuggerAgent.

src/execution.cc

Index: src/execution.cc
diff --git a/src/execution.cc b/src/execution.cc
index b88cdd54353f0fab1a3ba53d38a5f4a9062b33aa..8f8c1530436f5c3eecb8546248ebfb765f64ad12 100644
--- a/src/execution.cc
+++ b/src/execution.cc
@@ -714,6 +714,9 @@ void Execution::DebugBreakHelper(Isolate* isolate) {
 
 void Execution::ProcessDebugMessages(Isolate* isolate,
                                      bool debug_command_only) {
+  // Assert that we are on the main thread of the isolate.
+  ASSERT(ThreadId::Current().Equals(isolate->thread_id()));
+
   isolate->stack_guard()->ClearDebugCommand();
 
   StackLimitCheck check(isolate);