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Useful for multi-thread or multi-process stack trace printing, when
competing threads step on each other's stack traces. For example:

base: :debug::StackTrace(50).Print(" [" + std::toString(getpid()) + "] ");
Change-Id: I6d1e686b22769de4a58c7232f93d3a9ca0a1ff0f
Reviewed-on: https://chromium-review.googlesource.com/1199698
Commit-Queue: Mason Freed <masonfreed@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589956}