Add mojo package manager skeleton.

mojo/tools/package_manager/BUILD.gn

Index: mojo/tools/package_manager/BUILD.gn
diff --git a/mojo/examples/png_viewer/BUILD.gn b/mojo/tools/package_manager/BUILD.gn
similarity index 50%
copy from mojo/examples/png_viewer/BUILD.gn
copy to mojo/tools/package_manager/BUILD.gn
index 3fc0360e2398a62e0e7fce1f1f6f81302657c74a..466bc52f5c4bd7ef20a840d178111b1726765843 100644
--- a/mojo/examples/png_viewer/BUILD.gn
+++ b/mojo/tools/package_manager/BUILD.gn
@@ -2,24 +2,26 @@
 # 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",
   ]
 }