Handle iframe.src with a javascript protocol URL.

This change prevents us from crashing on Amazon and other pages with
iframe.src="javascript:foo".
Make all iframes attached to a document default to about:blank.
Update PowerMeasure.py

fixing a bug in displaying results
Merge pull request #3349 from LalehB/patch-3

Update PowerMeasure.py
Merge pull request #3299 from servo/iframe-javascript-urls

Handle iframe.src with a javascript: URL.
Merge pull request #3347 from jdm/blankiframe

Make all iframes attached to a document default to about:blank.