Bug 853508 - Lazy load SelectionHandler. r=bnicholson
Bug 852773: Remove assertions that make us horribly slow when debugging JS in a DEBUG build. r=jandem

When running JS code under the JS debugger in a DEBUG build,
Debugger::slowPathOnEnterFrame constructs a ScriptFrameIter instance every
time we enter a function. That constructor calls (JaegerMonkey's)
ExpandAllInlineFrames, which is very expensive.

We need Debugger to work in DEBUG builds, too.