mw.ViewPageTarget: Use native .focus() rather than jQuery's .focus()

jQuery's implementation does some sort of traversal, so on large pages
it took 300-400ms just to focus the document node.

Change-Id: I0de95d8800a1aedeb19b27d204dfd8fd08cec533
Merge "mw.ViewPageTarget: Use native .focus() rather than jQuery's .focus()"