id of iframe incorrectly sets window name

As per the specification, the iframe's name should be an empty string if unset:
http://www.whatwg.org/specs/web-apps/current-work/multipage/the-iframe-element.html#attr-iframe-name

Instead, Chromium was using the iframe's id as window name if the name was
unset. This change makes us behave like Firefox 27 and IE 11, verified using:
http://jsfiddle.net/xf5H7/9/

This CL sets the name attribute iframe in a lot of layout tests so that
testRunner.dumpChildFramesAsText() keeps printing the same result.

R=arv@chromium.org, tkent@chromium.org, abarth, arv, tkent
BUG=347169
TEST=fast/frames/iframe-no-name.html

Review URL: https://codereview.chromium.org/187103002

git-svn-id: svn://svn.chromium.org/blink/trunk@168553 bbb929c8-8fbe-4397-9dbb-9b2b20218538