Add mojo package manager skeleton.

mojo/tools/package_manager/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_examples.gypi:mojo_png_viewer
-shared_library("png_viewer") {
-  output_name = "mojo_png_viewer"
+shared_library("package_manager") {
+  output_name = "mojo_package_manager"
 
   sources = [
-    "png_viewer.cc"
+    "package_fetcher.cc",
+    "package_fetcher.h",
+    "package_manager.cc",
+    "package_manager_application.cc",
+    "package_manager_application.h",
+    "unpacker.cc",
+    "unpacker.h",
   ]
 
   deps = [
+    "//base",
     "//mojo/application",
-    "//mojo/examples/media_viewer:bindings",
     "//mojo/public/c/system:for_shared_library",
-    "//mojo/public/cpp/bindings",
     "//mojo/public/cpp/utility",
-    "//mojo/services/public/cpp/view_manager",
-    "//mojo/services/public/interfaces/content_handler",
     "//mojo/services/public/interfaces/network",
-    "//skia",
-    "//ui/gfx",
+    "//third_party/zlib:zip",
+    "//url",
   ]
 }