[Sampling profiler] Encapsulate Mac stack walking 4/4: unw_init_local workaround

The platform-native stack walking needs to be encapsulated into a function that
can be invoked in an alternating fashion with V8 unwinding, once supported. This
change inlines the workaround for avoiding a crash in unw_init_local into the
core stack walking function. The encapsulation requires inlining
otherwise-sensible abstractions to enable creation of future abstractions that
can be used across platforms. No functional change is intended.

Bug: 931418
Change-Id: I89065f8afa2e88f6a263880e0f7bb8ed1c5a6c5f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1513210
Commit-Queue: Mike Wittman <wittman@chromium.org>
Reviewed-by: Charlie Andrews <charliea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641377}