This CL prepares support for memory operations, where we will need to
call runtime functions for trapping, and hence need safepoint
information for iterating the frames correctly.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I8c65d80cd69747d13d9084e7427253413fb4b808
Reviewed-on: https://chromium-review.googlesource.com/810784
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49952}