Move wgBreakFrames client code to mediawiki.page.startup

Move wgBreakFrames client code to mediawiki.page.startup

Follows-up f7c324685195, which migrated this from legacy wikibits
to the 'mediawiki.page.ready', however it's better suited in
'mediawiki.page.startup' because that one loaded on all pages blindly
(used to be hardcoded in OutputPage, now part of 'core' group
in Skin::getDefaultModules).

mediawiki.page.ready on the other hand is primarily for enhancing
the page content, loaded in Skin::getDefaultModules in the 'content'
group, which extensions like MobileFrontend may override with an
alternate implementation. This means frame breaking is bypassed!

Change-Id: Ia7206fac5c4ec6ace87304cfaeef375916b94fcf