Use the mojo::shell namespace consistently under shell/.

(Esp. under shell/application_manager/.)

R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/972433004
diff --git a/shell/android/background_application_loader.h b/shell/android/background_application_loader.h
index 8baacbb..9dcee40 100644
--- a/shell/android/background_application_loader.h
+++ b/shell/android/background_application_loader.h
@@ -13,6 +13,7 @@
 #include "shell/application_manager/application_loader.h"
 
 namespace mojo {
+namespace shell {
 
 class BackgroundApplicationLoader
     : public ApplicationLoader,
@@ -57,6 +58,7 @@
   DISALLOW_COPY_AND_ASSIGN(BackgroundApplicationLoader);
 };
 
+}  // namespace shell
 }  // namespace mojo
 
 #endif  // SHELL_ANDROID_BACKGROUND_APPLICATION_LOADER_H_