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

blimp/client/core/render_widget/BUILD.gn

Index: blimp/client/core/render_widget/BUILD.gn
diff --git a/blimp/client/core/render_widget/BUILD.gn b/blimp/client/core/render_widget/BUILD.gn
index 2448725bbf9a7bd45d4b2b2794442fe7f0af80cb..57d7d919bcc20f7801106e78021dced7a6bb4a39 100644
--- a/blimp/client/core/render_widget/BUILD.gn
+++ b/blimp/client/core/render_widget/BUILD.gn
@@ -3,7 +3,10 @@
 # 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 = [
     "blimp_document.cc",
@@ -28,6 +31,12 @@ source_set("render_widget") {
 }
 
 source_set("test_support") {
+  visibility = [
+    "//blimp/client/core/render_widget:unit_tests",
+    "//blimp/client/core/compositor:unit_tests",
+    "//blimp/engine:browser_tests",  # # TODO(nyquist): Remove this. See crbug/653789.
+  ]
+
   testonly = true
 
   sources = [