When an HTML link has a target that is open already, clicking on the link should re-focus the tab.

Minimal repro: http://output.jsbin.com/tuducot/

There was a crack here that was exposed through User Activation v2 but never covered in any tests.  The crack got fixed in crrev.com/c/1006047 but we need a test.

FYI: WebViewTest.FocusExistingFrameOnNavigate incompletely covers this case.  While this test correctly asserts that DidFocus gets called, the TestWebViewClient in this test isolates (mocks) the browser-side focusing mechanism (in particular, dispatching of ViewHostMsg_Focus).