Default measurement should always be global

[aomarks]

Currently the default measurement is bench.start/stop when using the built-in server, and first-contentful-paint when using an external URL.

FCP is a particularly bad default, because if you implement either of the other two measurement styles in your code, FCP will still work and return some number, and it's easy to think you are measuring what you implemented instead of FCP.

Let's make the window.tachometerResult global the default in all cases, since it's the most universal (works in all browsers and with both internal/external URLs).

Also, the CLI flag is called "measure", but the JSON config file property is "measurement". These should be the same.