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This CL introduces HeapTracer which a single concept to track interesting
garbage collection numbers across garbage collections.

This allows for unit testing measurements and unifying the histogram
logic into a single place.

Bug: chromium:840789
Change-Id: Ie36d6f9cc973f0dec28a3305b2d0def59bafe135
Reviewed-on: https://chromium-review.googlesource.com/1046647
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556885}