Add a new permission kRead to PageAllocator::Permission and
OS::MemoryPermission and implement it in platform-*.

Not used yet, because it needs corresponding changes in chromium.

Bug: v8:7464
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I9f84251eff593536cbcc1cde04641d696c79d65c
Reviewed-on: https://chromium-review.googlesource.com/1006756Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52636}