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

shell/application_manager/shell_impl.h

Index: shell/application_manager/shell_impl.h
diff --git a/shell/application_manager/shell_impl.h b/shell/application_manager/shell_impl.h
index 5b44e9ad06fb1a4ac57d3684285666ce2c156858..5f405e5cff8f208af56bd65c0b2d5e200415f2ba 100644
--- a/shell/application_manager/shell_impl.h
+++ b/shell/application_manager/shell_impl.h
@@ -12,6 +12,8 @@
 #include "url/gurl.h"
 
 namespace mojo {
+namespace shell {
+
 class ApplicationManager;
 
 class ShellImpl : public Shell, public ErrorHandler {
@@ -53,6 +55,7 @@ class ShellImpl : public Shell, public ErrorHandler {
   DISALLOW_COPY_AND_ASSIGN(ShellImpl);
 };
 
+}  // namespace shell
 }  // namespace mojo
 
 #endif  // SHELL_APPLICATION_MANAGER_SHELL_IMPL_H_