Adds support to the CPU profiler for scraping the incumbent contexts of
V8 stack frames. While it is generally unsafe to access heap objects
during a profiling interrupt, the native context is uniquely usable due
to being guaranteed an alive root on the stack, as well as its slots
being immutable after context creation.

Change-Id: I2c3149c1302b74d2f13aa99d1fdd0cf006e0f9d1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1580020
Commit-Queue: Andrew Comminos <acomminos@fb.com>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Alexei Filippov <alph@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63258}