Don't throw on non-zero exit in mach commands

Fixes #3344.
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.
Merge pull request #3345 from mbrubeck/mach-exception

Don't throw on non-zero exit in mach commands
Update PowerMeasure.py

fixing a bug in displaying results
Add viewer for layout trace files.
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.
Update PowerMeasure.py
Added a bloom filter to CSS selector matching.
Merge pull request #3347 from jdm/blankiframe

Make all iframes attached to a document default to about:blank.
Merge pull request #3355 from LalehB/patch-4

Update PowerMeasure.py
Handle src='' in an iframe element. Without this, infinitely creates iframes with the same url.
Merge pull request #3212 from cgaebel/style-resolution-bloom-filter

Added a bloom filter to CSS selector matching.
Fix font flickering due to missed check in font cache.
Merge pull request #3362 from glennw/fix-layout-fonts

Fix font flickering due to missed check in font cache.
Move link rel=stylesheet fetching to layout task

Fixes #3346.
Merge pull request #3357 from glennw/empty-iframe-src

Handle src='' in an iframe element. Without this, infinitely creates iframes with the same url.
Merge pull request #3352 from mbrubeck/link-style

 Move link rel=stylesheet fetching to layout task. r=jdm
Implement percentage heights for block flows

Fixes #2029