Adds a ReadOnlyRoots class trivially constructable from a Heap* or
Isolate* and which can be obtained from a any HeapObject which provides
access to roots objects that will always be in RO_SPACE. In the longer
term this object will be accessed via a global variable without
requiring an Isolate or using the memory address of a HeapObject to
infer it.

Moves the list macros in heap.h to roots.h and splits some of them into
two parts (read-only and mutable).

Convert cases of heap_object->GetHeap()->root_accessor() to
heap_objects->GetReadOnlyRoots().root_accessor().

Bug: v8:7786
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I14b01052adb2af9a5ec82b970e933d6a423d17a5
Reviewed-on: https://chromium-review.googlesource.com/1122127
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54160}