src/chrome/renderer/extensions/user_script_slave.cc:79
  UserScriptSlave::GetIsolatedWorldIdForExtension(..)
    // Calls InitializeIsolatedWorld(new_id, extension);
// Called on two locations:
// Dynamic executeScript ?
src/chrome/renderer/extensions/user_script_scheduler.cc
  void UserScriptScheduler::ExecuteCodeImpl(
    int isolated_world_id =
      dispatcher_->user_script_slave()->GetIsolatedWorldIdForExtension(
        extension, child_frame);
    // few lines ahead
    executeScriptInIsolatedWorld( ... )
    // in src/third_party/WebKit/Source/web/WebFrameImpl.cpp
    // WebFrameImpl::executeScriptInIsolatedWorld
    // calls
    // src/third_party/WebKit/Source/bindings/v8/ScriptController.cpp
    ScriptController::executeScriptInIsolatedWorld
    // Initializes:
    V8WindowShell* isolatedWorldShell = windowShell(world.get())
    // V8WindowShell* ScriptController::windowShell(DOMWrapperWorld* world)
    // calls:
    if (!shell->isContextInitialized() && shell->initializeIfNeeded())
    // bool V8WindowShell::initializeIfNeeded()
    // calls
    bool V8WindowShell::initialize()
    // calls
    m_frame->loader().client()->didCreateScriptContext(context, m_world->extensionGroup(), m_world->worldId());
    // ^ ultimately calls  Dispatcher::DidCreateScriptContext.
// From manifest
src/chrome/renderer/extensions/user_script_slave.cc
  void UserScriptSlave::InjectScripts(WebFrame* frame,
                                      UserScript::RunLocation location) {
      int isolated_world_id = GetIsolatedWorldIdForExtension(extension, frame);
    // Few lines ahead
    frame->executeScriptInIsolatedWorld(
              isolated_world_id, &sources.front(), sources.size(),
    // Same code path as the other thing
:set invnumber