No longer access the heap directly, as policed by Disallow*
scopes in JSContextSpecialization::Reduce.

Bug: v8:7790
Change-Id: I40f1c500b04b96152421fd5de631747ba386bca1
Reviewed-on: https://chromium-review.googlesource.com/1101322
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53759}