[ios] Adds a CommandDispatcher to proxy method calls to UI handlers.

ios/clean/chrome/browser/ui/commands/BUILD.gn

 # Copyright 2016 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.
 
 source_set("commands") {
   sources = [
+    "command_dispatcher.h",
+    "command_dispatcher.mm",
     "settings_commands.h",
     "tab_commands.h",
     "tab_grid_commands.h",
     "toolbar_commands.h",
   ]
+  deps = [
+    "//base",
+  ]
 
   configs += [ "//build/config/compiler:enable_arc" ]
 }
+
+source_set("unit_tests") {
+  testonly = true
+  sources = [
+    "command_dispatcher_unittest.mm",
+  ]
+  deps = [
+    ":commands",
+    "//base",
+    "//testing/gtest",
+  ]
+
+  configs += [ "//build/config/compiler:enable_arc" ]
+}