Temporarily reducing precision of DOMHighResTimeStamp in Chrome

As part of our mitigations against Speculative Side Channel Attacks
<https://www.chromium.org/Home/chromium-security/ssca>, Chrome 64 will
reduce the resolution of DOMHighResTimeStamps to 100us. We are also
introducing 100us of random jitter to the clock edges to prevent attempts
to increase resolution via edge-thresholding.  The returned time will be
within +/100us of real time, however there will be no guarantee as to
duration (in real time) of clock pulses between these 100us intervals.

We intend for this to be a temporary measure while other mitigation are
introduced, however we do not have a timeline for restoring the previous
precision.

Ross / Sami