Do the early return when role is different than UnknownRole
Do the early return when role is different than UnknownRole

We do not need to call ariaRoleAttribute() to check whether current role
is different than UnknownRole. Just check m_ariaRole instead.

BUG=

Review URL: https://codereview.chromium.org/756823004

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@186527 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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