Introduce StackProfileBuilder to generate a CallStackProfile

This CL introduces a new class StackProfileBuilder owned by StackSamplingProfiler.

This CL also refactors both Windows and Mac profile construction into the
StackProfileBuilder class to generate a StackSamplingProfiler::CallStackProfile.

Bug: 851163
Change-Id: I6acde44fc1e38ae04d1d747fe2d3843faba3a218
Reviewed-on: https://chromium-review.googlesource.com/1101462
Reviewed-by: Mike Wittman <wittman@chromium.org>
Commit-Queue: Xi Cheng <chengx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568239}
6 files changed