Add a sampling trigger to collect memlog reports for a control population

This CL is adding a trigger that randomly collect a sample memlog report.
These reports are used to compare high-memory usage reports with
common memory usage reports.

R=erikchen
BUG=867921

Change-Id: I2a05e0dac1184f34ffc32978cbcfc41170a5b665
Reviewed-on: https://chromium-review.googlesource.com/1151406
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578340}
3 files changed