Move process_milestone to CallStackProfileBuilder class

Bug: 851163
Change-Id: I24a4a56941346f940b35c8add937163788bab7c5
Reviewed-on: https://chromium-review.googlesource.com/1125131
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Mike Wittman <wittman@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Xi Cheng <chengx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572687}
11 files changed