Do not beep on failed copy/cut.

WebCore::systemBeep() is only used by WebCore::Editor and
WebCore::EditorCommand, so we silence it.

BUG=36963

Review URL: https://chromiumcodereview.appspot.com/24569002

git-svn-id: svn://svn.chromium.org/blink/trunk@158534 bbb929c8-8fbe-4397-9dbb-9b2b20218538