Disable the focus moves to an element which doesn't satisfy the spec.


The current behavior, if there are some elements that are set ‘ tabindex="negative integer" ', and then, when one of them is focused on, the next focus moves to the others.
however, it does not match the spec.
This patch disallow them.

The spec:
 http://www.w3.org/TR/html5/editing.html#sequential-focus-navigation-and-the-tabindex-attribute


BUG=395761

Review URL: https://codereview.chromium.org/484383002

git-svn-id: svn://svn.chromium.org/blink/trunk@182040 bbb929c8-8fbe-4397-9dbb-9b2b20218538