Remove Document::m_ignoreAutofocus which is never set.

No reachable codepath sets m_ignoreAutofocus and it seems like it's
been this way since 2011.

The idea of m_ignoreAutofocus was to prevent autofocus from taking focus
if the user was already typing in a text field. It was added with the
patch to implement autofocus in 2008:
http://trac.webkit.org/changeset/34626

The only codepath that could have potentially set m_ignoreFocus was
removed in 2011: http://trac.webkit.org/changeset/91014 and there was a
bug in 2009 that seems to indicate that code was never entered:
https://bugs.webkit.org/show_bug.cgi?id=31032

So it seems the functionality hasn't worked for several years. No
layout test was added for it in the original patch. We can remove
the boolean and reimplement if we want to support it in the future.

Review URL: https://codereview.chromium.org/25681002

git-svn-id: svn://svn.chromium.org/blink/trunk@158698 bbb929c8-8fbe-4397-9dbb-9b2b20218538