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This adds code to query the process' debug symbol maps for offsets
and produces an externally symbolizable backtrace using that data.

R: thakis@chromium.org,wez@chromium.org
CC: jamesr@chromium.org
Bug: 706592
Change-Id: I7b3257ff154671b2eb1c90f7ab91947f55cfa873
Reviewed-on: https://chromium-review.googlesource.com/575290
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491470}