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

blimp/client/core/context/BUILD.gn

Index: blimp/client/core/context/BUILD.gn
diff --git a/blimp/client/core/context/BUILD.gn b/blimp/client/core/context/BUILD.gn
index 5b767d7b68f6fc1656503659b6920a3cf3c8da97..737e8db137e3c2c4f02202e342cf12cfd173427f 100644
--- a/blimp/client/core/context/BUILD.gn
+++ b/blimp/client/core/context/BUILD.gn
@@ -8,6 +8,13 @@ if (is_android) {
 }
 
 source_set("context") {
+  visibility = [
+    ":unit_tests",
+    "//blimp/client/app:session",  # TODO(nyquist): Remove this. See crbug/651964.
+    "//blimp/client/core",
+    "//blimp/engine:browser_tests",  # TODO(nyquist): Remove this. See crbug/653789.
+  ]
+
   sources = [
     "blimp_client_context_impl.cc",
     "blimp_client_context_impl.h",
@@ -71,6 +78,8 @@ source_set("dummy") {
 }
 
 source_set("unit_tests") {
+  visibility = [ "//blimp/client/core:unit_tests" ]
+
   testonly = true
 
   sources = [
@@ -87,7 +96,7 @@ source_set("unit_tests") {
 
 if (is_android) {
   android_library("context_java") {
-    visibility = [ "//blimp/client/*" ]
+    visibility = [ "//blimp/client/core/*" ]
 
     java_files = [ "android/java/src/org/chromium/blimp/core/BlimpClientContextImpl.java" ]
 
@@ -104,7 +113,7 @@ if (is_android) {
   }
 
   android_library("dummy_context_java") {
-    visibility = [ "//blimp/client/*" ]
+    visibility = [ "//blimp/client/core/*" ]
 
     java_files = [ "android/java/src/org/chromium/blimp/core/DummyBlimpClientContext.java" ]