Remove WebCore::applyCommand and just call apply() directly

This function doesn't do anything except call apply(), so we might as well
do that directly. There's already several call sites that were doing this
anyway.

Review URL: https://chromiumcodereview.appspot.com/23451009

git-svn-id: svn://svn.chromium.org/blink/trunk@157231 bbb929c8-8fbe-4397-9dbb-9b2b20218538