[Mac] Remove base::LaunchSynchronize and rewrite content::MachBroker.

base/mac/scoped_mach_port.cc

Index: base/mac/scoped_mach_port.cc
diff --git a/base/mac/scoped_mach_port.cc b/base/mac/scoped_mach_port.cc
index 652e3f44852161b4f69e0754855b1de93451e2c6..9e45a856a8cf8b38662def3c761f8e05743c17d5 100644
--- a/base/mac/scoped_mach_port.cc
+++ b/base/mac/scoped_mach_port.cc
@@ -11,9 +11,14 @@ ScopedMachPort::ScopedMachPort(mach_port_t port) : port_(port) {
 }
 
 ScopedMachPort::~ScopedMachPort() {
+  reset();
+}
+
+void ScopedMachPort::reset(mach_port_t port) {
   if (port_ != MACH_PORT_NULL) {
     mach_port_deallocate(mach_task_self(), port_);
   }
+  port_ = port;
 }
 
 }  // namespace mac