Move session into //blimp/client/app and update GN files.

blimp/client/core/session/BUILD.gn

 # Copyright 2016 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.
 
 if (is_android) {
   import("//build/config/android/config.gni")
   import("//build/config/android/rules.gni")
 }
 
 source_set("session") {
   visibility = [
-    "//blimp/client/*",
+    "//blimp/client/app:*",  # TODO(nyquist): Remove this. See crbug/651964.
+    "//blimp/client/core/*",
     "//blimp/engine:browser_tests",
   ]
 
   sources = [
     "assignment_source.cc",
     "assignment_source.h",
     "client_network_components.cc",
     "client_network_components.h",
     "connection_status.cc",
     "connection_status.h",
@@ skipping 53 matching lines @@
       "//net:test_support",
       "//url",
     ]
 
     data += [
       "test_selfsigned_cert.pem",
       "//blimp/test/data/test_client_token",
     ]
   }
 }