The affected passes are part of OptimizeGraph which runs on a background
thread, so they must not access the heap (cast has a SLOW_DCHECK that
looks at the object's type).

I'm also changing Handle<T>::cast to assert that it's called in a scope
allowing handle dereference.

R=mstarzinger@chromium.org

Bug: 
Change-Id: Ibae1e40ec8d57481387570c658eb7cb1ae5de3b9
Reviewed-on: https://chromium-review.googlesource.com/570403Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46667}