Pass the selection text into Pasteboard from the Editor.

Also remove the ShouldSerializeSelectedTextForClipboard enum, as only
one of its enumerators ever applies.

BUG=276565

Review URL: https://chromiumcodereview.appspot.com/23242013

git-svn-id: svn://svn.chromium.org/blink/trunk@156569 bbb929c8-8fbe-4397-9dbb-9b2b20218538