Blimp: create MessageDispatcher class.

blimp/BUILD.gn

Index: blimp/BUILD.gn
diff --git a/blimp/BUILD.gn b/blimp/BUILD.gn
index 9f601ad168c12aa89940c8af90dcce500a051f1d..550c24568991a5ab90eb5e33ebb006ca6bcd3cd5 100644
--- a/blimp/BUILD.gn
+++ b/blimp/BUILD.gn
@@ -2,6 +2,7 @@
 # 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",
@@ -12,3 +13,11 @@ group("blimp") {
     deps += [ "//blimp/client:blimp_apk" ]
   }
 }
+
+# Metatarget for all of Blimp's unit tests.
+group("blimp_tests") {
+  testonly = true
+  deps = [
+    "//blimp/common:blimp_common_unit_tests",
+  ]
+}