Handle iframe.src with a javascript protocol URL.

This change prevents us from crashing on Amazon and other pages with
iframe.src="javascript:foo".
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.
Move link rel=stylesheet fetching to layout task

Fixes #3346.
Refactor some code into resource_task