This encapuslates some of the heap accesses done by
JSNativeContextSpecialization::ReduceJSLoadGlobal and
JSNativeContextSpecialization::ReduceJSStoreGlobal.

Bug: v8:7790
Change-Id: Ib6c63903809927d6094af22519285cb9d0bbff7a
Reviewed-on: https://chromium-review.googlesource.com/1106141Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53845}