Extract a Connector interface from Shell that can be cloned & passed to other threads

mojo/shell/public/cpp/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # GYP version: mojo/mojo_base.gyp:mojo_application_base
 source_set("cpp") {
   public_deps = [
     ":init_commandline",
     ":sources",
   ]
 }
 
 # Like the target above, but without special commandline initialization that
 # apps use.
 source_set("cpp_for_chromium") {
   public_deps = [
     ":sources",
   ]
 }
 
 source_set("sources") {
   sources = [
     "application_runner.h",
     "connect.h",
     "connection.h",
+    "connector.h",
     "initialize_base_and_icu.cc",
     "interface_binder.h",
     "interface_factory.h",
     "interface_factory_impl.h",
     "interface_registry.h",
     "lib/application_runner.cc",
     "lib/connection_impl.cc",
     "lib/connection_impl.h",
+    "lib/connector_impl.cc",
+    "lib/connector_impl.h",
     "lib/interface_factory_binder.h",
     "lib/interface_registry.cc",
     "lib/shell_client.cc",
     "lib/shell_connection.cc",
     "shell.h",
     "shell_client.h",
     "shell_connection.h",
   ]
 
   deps = [
@@ skipping 34 matching lines @@
     "//base/test:test_support",
     "//mojo/logging",
     "//mojo/public/cpp/bindings",
     "//mojo/public/cpp/environment",
     "//mojo/public/cpp/system",
     "//mojo/shell/public/interfaces:interfaces_cpp_sources",
   ]
 
   data_deps = []
 }