HTMLScriptElement::setText should use AttachLazily

In addition to switching setText to use AttachLazily I factored out the
checks for a single text node child used here and inside markup.cpp to
be shared.

Review URL: https://chromiumcodereview.appspot.com/22966008

git-svn-id: svn://svn.chromium.org/blink/trunk@156382 bbb929c8-8fbe-4397-9dbb-9b2b20218538