1.9.1 beta 2 checkins

There are a lot of things queuing up to check in for the 1.9.1 beta 2 freeze. We want to make sure that there's adequate test coverage for everything, so we know what to back out when tests fail. Please sign up below for a chance to push.

Name	Description/Bug#
bent	js exceptions off main thread/web worker spec changes
smaug	bug 454108
roc	Trivial bug 462972, nontrivial image snapping changes bug 458487, low-risk column changes bug 462307
mak77	bug 445737, bug 418643
dietrich	bug 419792, bug 457473, bug 407110
dao	bug 436304
gavin	bug 235853 PAC bug, bug 461803 elementFromPoint changes, bug 17612 view-source link browsing front-end patch (very safe)