... and Heap::root_handle() to RootsTable.

This is a preliminary step before moving IsolateData object from Heap to Isolate
which is required for pointer-compression friendly heap layout.

Bug: v8:8182
Change-Id: Ideacc1c9e4435be7a33db08415ac1ad46e956199
Reviewed-on: https://chromium-review.googlesource.com/c/1273238Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56552}