Removing redundant names introduced for input type checkbox with menu as parent and aria role menuitemcheckbox

In this CL https://codereview.chromium.org/589383004/, a redundant name was introduced
for input type checkbox with menu as parent. Rather the role defined for aria
role menuitemcheckbox can be used for input type checkbox with menu as parent.

Review URL: https://codereview.chromium.org/649073002

git-svn-id: svn://svn.chromium.org/blink/trunk@183591 bbb929c8-8fbe-4397-9dbb-9b2b20218538