Turbofan does not always maintain source positions. Since the profiler
is best-effort only, this is ok and we should not assert it.
Source positions for stack traces are computed with an independent
mechanism: the stack walker. It's impossible to use the stack walker
here since it assumes that the frames actually exist.

Bug: chromium:787959
Change-Id: I43c3c4e6de0a9b5f76b05c53af238cb1df6ea1c5
Reviewed-on: https://chromium-review.googlesource.com/822930Reviewed-by: Alexei Filippov <alph@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50053}