ContentDistributor::collectSelectFeatureSetFrom only needs to travserse the descendants once

collectSelectFeatureSetFrom was walking all of the descendants of the ShadowRoot twice if you
had both a <content> and a <shadow>. Instead we can just walk once.

R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/19956006

git-svn-id: svn://svn.chromium.org/blink/trunk@154861 bbb929c8-8fbe-4397-9dbb-9b2b20218538