Use ElementTraversal API in siblingWithAriaRole() to simplify the code

Use ElementTraversal API in siblingWithAriaRole() to simplify the code a bit.

R=adamk, dmazzoni@chromium.org

Review URL: https://codereview.chromium.org/200473005

git-svn-id: svn://svn.chromium.org/blink/trunk@169280 bbb929c8-8fbe-4397-9dbb-9b2b20218538