c++ - In Chrome V8 Engine Source, What it means for?

I have a question in following piece of code.

#define V8_DECLARE_ONCE(NAME) ::v8::base::OnceType NAME

In cpp file, '::' means for referencing namespace, but where '::v8' included?

std::cout << ... << std::endl;

cout is under the 'std' namespace,,, but, in this case, i have no idea how to interpret it.

part of this code is following one.

 namespace v8 {
 namespace base {

 typedef AtomicWord OnceType;

 #define V8_ONCE_INIT 0

 #define V8_DECLARE_ONCE(NAME) ::v8::base::OnceType NAME