Focus unfocusable node upon reference fragment navigation

Currently, when the page navigates to a non-focusable reference fragment,
the currently focused element does not change. In Firefox/IE, the focus is
moved to the referenced element even when this element is non-focusable.

This mismatch in behavior became apparent when Blink started supporting
a:focus (https://crbug.com/388666). In IE/FF, clicking on <a href="#ref">
causes the link to loose focus, and therefore :focus does not get applied
any more. In Chrome, however, :focus remains activated because the focus
does not change.

This patch changes the behavior of Blink: When the pages navigates to a
reference fragment that refers to a non-focusable node, then the focus
is removed from the previously focused element.

See also https://www.w3.org/Bugs/Public/show_bug.cgi?id=26907.

BUG=417636,262171
TEST=Tools/Scripts/run-webkit-tests fast/dom/fragment-activation-focuses-target.html

Review URL: https://codereview.chromium.org/637323002

git-svn-id: svn://svn.chromium.org/blink/trunk@183455 bbb929c8-8fbe-4397-9dbb-9b2b20218538