We'll soon start collecting data from the JS heap prior to the typed
lowering pass, and then refrain from reading the heap in that pass.
This CL prepares the broker machinery by introducing a hash table that
maps an object (handle) to the corresponding cached data. For the time
being, that cached data is essentially just the handle itself.

Bug: v8:7790
Change-Id: I830e9c72faafb7ae1d10e8a111636b3a3762bbc6
Reviewed-on: https://chromium-review.googlesource.com/1143405
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54618}