Add <root>_handle getters to ReadOnlyRoots, so an Isolate isn't required
to get a handle of a root.

Also convert one GetIsolate()->factory()->accessor() to
GetReadOnlyRoots().accessor_handle() in FixedArray.

Bug: v8:7786
Change-Id: I1b72c58f90eacf64be95abb65df91cf993178a93
Reviewed-on: https://chromium-review.googlesource.com/1129225
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54334}