When iterating over stack frames in the cpu profiler, don't perform any
object casts that have heap-testing DCHECKs. Instead, access values on
the frame by offsets directly, and only check their tags for validity.

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Ia54b18f8ab947c1827f17483806104f0d1d34136
Reviewed-on: https://chromium-review.googlesource.com/536973
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45985}