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/application_manager/application_manager.h b/shell/application_manager/application_manager.h
index 844be55..52b0ec6 100644
--- a/shell/application_manager/application_manager.h
+++ b/shell/application_manager/application_manager.h
@@ -27,6 +27,7 @@
 }
 
 namespace mojo {
+namespace shell {
 
 // ApplicationManager requires implementations of NativeRunner and
 // NativeRunnerFactory to run native applications.
@@ -260,6 +261,7 @@
   DISALLOW_COPY_AND_ASSIGN(ApplicationManager);
 };
 
+}  // namespace shell
 }  // namespace mojo
 
 #endif  // SHELL_APPLICATION_MANAGER_APPLICATION_MANAGER_H_