Fix button-disabled-blur.html.

This is a follow-up of Blink r165275 [1]. r165275 made the test flaky. The test
doesn't need to use autofocus

[1] http://src.chromium.org/viewvc/blink?view=revision&revision=165275

BUG=324707
TBR=keishi@chromium.org

Review URL: https://codereview.chromium.org/141573003

git-svn-id: svn://svn.chromium.org/blink/trunk@165292 bbb929c8-8fbe-4397-9dbb-9b2b20218538