Enable cross-origin isolation to improve timing precision

[nolanlawson]

By adding the HTTP headers to enable cross-origin isolation, it should be possible to get higher-precision timing from APIs like performance.now() and performance.mark()/measure(), at least in Chromium and Firefox.

This ought to help Tachometer provide more stable results. Also, since the Tachometer benchmark site doesn't pull in any cross-origin resources (AFAIK), it should be a non-breaking change.

[nolanlawson]

As a bonus, enabling cross-origin isolation also unlocks APIs like performance.measureUserAgentSpecificMemory and SharedArrayBuffer. I can't think of any existing use cases that would be broken by enabling it, either.