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/shell_impl.cc b/shell/application_manager/shell_impl.cc
index 677e84e..289b19b 100644
--- a/shell/application_manager/shell_impl.cc
+++ b/shell/application_manager/shell_impl.cc
@@ -9,6 +9,7 @@
 #include "shell/application_manager/application_manager.h"
 
 namespace mojo {
+namespace shell {
 
 ShellImpl::ShellImpl(ApplicationPtr application,
                      ApplicationManager* manager,
@@ -56,4 +57,5 @@
   manager_->OnShellImplError(this);
 }
 
+}  // namespace shell
 }  // namespace mojo