[scheduler] Adding a flag to enable random sampling only when needed and supported.

The SequenceManagerImpl backend used calls to Random() to record metrics, which
is not supported on some platforms. Now we enable it only for Blink.

Bug: 891670
Change-Id: I487cc11960d141cff5861d1a2ca7fa09be2a866f
Reviewed-on: https://chromium-review.googlesource.com/c/1353921
Commit-Queue: Karolina Soltys <ksolt@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613970}
15 files changed