We should explain why we default to StackFrame::OPTIMIZED rather
than StackFrame::INTERPRETED when we don't have a great idea
about what kind of frame we are looking at.

Bug: 
Change-Id: I9d3cc84def6164ef79a4792822d52c11dfe8a448
Reviewed-on: https://chromium-review.googlesource.com/668450Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48565}