Drop all uses of //base except ObserverList from view mgr client lib

mojo/services/view_manager/public/cpp/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.
 
 import("../../../mojo_sdk_root.gni")
 import("$mojo_sdk_root/mojo/public/mojo_sdk.gni")
 
 mojo_sdk_source_set("cpp") {
   restrict_external_deps = false
   public_configs = [ "../../../public/build/config:mojo_services" ]
@@ skipping 27 matching lines @@
     "../../../input_events/public/interfaces",
     "../../../surfaces/public/interfaces:surface_id",
     "../../../window_manager/public/interfaces",
     "//base",
   ]
 
   mojo_sdk_deps = [
     "mojo/public/c/gles2:headers",
     "mojo/public/cpp/application",
     "mojo/public/cpp/bindings:bindings",
+    "mojo/public/cpp/system",
     "mojo/public/interfaces/application",
   ]
 }
 
 source_set("common") {
   sources = [
     "types.h",
   ]
 }