Fix fetching ParserOutput

WikiPage::getParserOutput() can return false when we don't hit
the parser cache and we're unable to parse due to something like
slave lag and our rev_id can't be found.

We also don't really want to use the pool counter here either, so just
request from the parser cache then fall back to Content::getParserOutput()
if we have to parse.

This is bug 53938 in a new form, which survived refactoring. You'd
think I would learn by now.

Bug: 58011
Change-Id: I3a148e92942f1b4f42cb53cebaf8abf34be6902b
Merge "Fix fetching ParserOutput"