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

blimp/client/core/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")
 }
 
 import("//blimp/client/core/config.gni")
@@ skipping 10 matching lines @@
     ]
   } else {
     deps = [
       ":dummy_core",
     ]
   }
 }
 
 group("core") {
   visibility = [
-    "//blimp/client/*",
+    ":core_shim",
+    "//blimp/client/app:*",  # TODO(nyquist): Remove this. See crbug/651964.
+    "//blimp/client/test",
     "//blimp/test/*",
   ]
 
   deps = [
     "//blimp/client/core/compositor",
     "//blimp/client/core/contents",
     "//blimp/client/core/context",
     "//blimp/client/core/feedback",
     "//blimp/client/core/geolocation",
     "//blimp/client/core/render_widget",
@@ skipping 39 matching lines @@
       ]
     } else {
       deps = [
         ":dummy_core_java",
       ]
     }
   }
 
   java_group("core_java") {
     visibility = [
-      "//blimp/client/*",
-      "//blimp:blimp_unittests_java_deps",
+      ":core_shim_java",
+      "//blimp/client:blimp_unittests_java_deps",
+      "//blimp/client/app:*",  # TODO(nyquist): Remove this. See crbug/651964.
     ]
 
     deps = [
       "//blimp/client/core/contents:contents_java",
       "//blimp/client/core/context:context_java",
       "//blimp/client/core/feedback:feedback_java",
       "//blimp/client/core/settings:settings_java",
       "//blimp/client/core/switches:switches_java",
       "//device/geolocation:geolocation_java",
     ]
   }
 
   java_group("dummy_core_java") {
     visibility = [ ":*" ]
 
     deps = [
       "//blimp/client/core/context:dummy_context_java",
     ]
   }
 }