Simplify code for finding body in FrameSelection::setSelectionFromNone()

Simplify code for finding body in FrameSelection::setSelectionFromNone() by
using Traversal<HTMLBodyElement>::firstChild(*documentElement). This is very
close to what is done in Document::body() except that body() searches for
both HTMLBodyElement and HTMLFrameSetElement. For this reason, we cannot
simply call Document::body() here.

Besides being shorter, the new code is also more optimal because:
- It does not check documentElement as it cannot be an HTMLBodyElement. For
  HTML documents, the documentElement is an HTMLHtmlElement.
- It avoids checking descendants and will only check the direct children of
  the <html> element.

R=adamk@chromium.org, leviw@chromium.org

Review URL: https://codereview.chromium.org/425393002

git-svn-id: svn://svn.chromium.org/blink/trunk@179259 bbb929c8-8fbe-4397-9dbb-9b2b20218538