Clamp performance.now() to 100us.

This patch reduces the resolution of performance.now() from 5us to 100us
and adds pseudorandom jitter on top.

TBR=skyostil@chromium.org

(cherry picked from commit a77687fd89adc1bc2ce91921456e0b9b59388120)

Authors: Ross McIlroy <rmcilroy@chromium.org>, Sami Kyostila <skyostil@chromium.org>
Bug: 798964
Change-Id: I7e401d7beae2efe8074cac8fc094b22e6dc857c2
Reviewed-on: https://chromium-review.googlesource.com/849993
Commit-Queue: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#527008}
Reviewed-on: https://chromium-review.googlesource.com/853505
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/branch-heads/3282@{#439}
Cr-Branched-From: 5fdc0fab22ce7efd32532ee989b223fa12f8171e-refs/heads/master@{#520840}
14 files changed