SECURITY: Copy prevent-clickjacking between OutputPage and ParserOutput

Special page transclusion returns an OutputPage, whose metadata is
copied into the ParserOutput, and then later back into an OutputPage.
The "preventClickjacking" flag should be part of that metadata.

Bug: 65778
Change-Id: I17d2720fb94bb383a92059e5adbf6c16ee3e9ef4