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

shell/android/android_handler.h

Index: shell/android/android_handler.h
diff --git a/shell/android/android_handler.h b/shell/android/android_handler.h
index c3716472987930b731e7f8dddca786b5b2d6fd5a..1e1ad969594ddc0bd6c6c82d7272706857f2414d 100644
--- a/shell/android/android_handler.h
+++ b/shell/android/android_handler.h
@@ -17,6 +17,7 @@ class FilePath;
 }
 
 namespace mojo {
+namespace shell {
 
 class AndroidHandler : public ApplicationDelegate,
                        public ContentHandlerFactory::Delegate {
@@ -39,6 +40,7 @@ class AndroidHandler : public ApplicationDelegate,
 
 bool RegisterAndroidHandlerJni(JNIEnv* env);
 
+}  // namespace shell
 }  // namespace mojo
 
 #endif  // MOJO_SHELL_ANDROID_CONTENT_HANDLER_H_