Bug 1173219 - Make TabParent::RecvCreateWindow less crash prone

Ok, this should be pretty straight-forward. I want to be able to send the nsresult back to the caller (success or failure), and then always return true from TabParent::RecvCreateWindow.

Easy-peasy.

There are a number of ways to do this… I've settled on having TabParent default to setting the aResult to NS_OK, and then setting to NS_ERROR_FAILURE in case things go wrong, and returning true. An alternative was to default it to NS_ERROR_FAILURE, and setting to in the success cases. Bah.

Interesting - sometimes we use no-braces for single-line conditionals where we normally used to do:

NS_ENSURE_SUCCESS(rv, false);

I did not know this.

try: https://treeherder.mozilla.org/#/jobs?repo=try&revision=086aae77d3c6

Ok, looking green! Landed!

This might fix a bunch of crashes we're seeing on Dev Edition, so requesting uplift.