Blimp: create MessageDispatcher class.

blimp/BUILD.gn

 # Copyright 2015 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.
 
+# Metatarget for all of Blimp's feature code.
 group("blimp") {
   deps = [
     "//blimp/client:blimp_client",
     "//blimp/common:blimp_common",
   ]
 }
+
+# Metatarget for all of Blimp's unit tests.
+group("blimp_tests") {
+  testonly = true
+  deps = [
+    "common:blimp_common_unit_tests",
+  ]
+}