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

blimp/client/core/render_widget/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.
 
 source_set("render_widget") {
-  visibility = [ "//blimp/client/*" ]
+  visibility = [
+    "//blimp/client/app:session",  # TODO(nyquist): Remove this. See crbug/651964.
+    "//blimp/client/core/*",
+  ]
 
   sources = [
     "render_widget_feature.cc",
     "render_widget_feature.h",
   ]
 
   deps = [
     "//blimp/common",
     "//cc/proto",
     "//net",
   ]
 
   public_deps = [
     "//base",
     "//blimp/net",
   ]
 }
 
 source_set("test_support") {
+  visibility = [
+    "//blimp/client/core/render_widget:unit_tests",
+    "//blimp/client/core/compositor:unit_tests",
+    "//blimp/engine:browser_tests",
+  ]
+
   testonly = true
 
   sources = [
     "mock_render_widget_feature_delegate.cc",
     "mock_render_widget_feature_delegate.h",
   ]
 
   public_deps = [
     ":render_widget",
     "//testing/gmock",
@@ skipping 14 matching lines @@
     ":test_support",
     "//base",
     "//blimp/common",
     "//blimp/net:test_support",
     "//cc/proto",
     "//net",
     "//net:test_support",
     "//testing/gtest",
   ]
 }