Makes ExternalReference count as a subclass of RawPtrT to enable
either to be passed to these functions as base argument.

BUG=v8:6949,v8:11074

Change-Id: I126856815ff7cdc0612e3c3fcdfdd4938cc19bfa
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2534820
Auto-Submit: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71171}