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

shell/application_manager/shell_impl.cc

Index: shell/application_manager/shell_impl.cc
diff --git a/shell/application_manager/shell_impl.cc b/shell/application_manager/shell_impl.cc
index 677e84ec924f9498c7f112086cc50b7c73b2d34c..289b19b85e7573dd44d9b6c86ff9340162f33fc0 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 @@ void ShellImpl::OnConnectionError() {
   manager_->OnShellImplError(this);
 }
 
+}  // namespace shell
 }  // namespace mojo