Bug 968857 - [SpecialPowers] Big files are truncated when loaded with loadChromeScript. r=ted, a=1.3+

With this patch, loadChromeScript now completely reads the input. This also
moves the loading code in its own function.

This also improves error reporting.
Bug 957391 - Call nsIFrame::SchedulePaint() in ImageLoader::DoRedraw(). r=mattwoodrow, a=1.3+
Bug 966802 - Fix and protect from stack overflow of the threads cloned in the Nuwa process. r=khuey, a=1.3+

Also re-enables the WebRTC tests disabled on emulator builds in bug 950266.