Intent to ship: PerformancePaintTiming API

[Sean Feng]

/Summary: /PerformancePaintTiming API allows developers to measure the
paint timing metric for their websites during pageload.
<https://en.wikipedia.org/wiki/Elevator_pitch>

/Bug/: 1518999

/Standard/: https://w3c.github.io/paint-timing/. There are two timing
entries defined in spec (`first-paint`(optional) and
`first-contentful-paint`). We'll only be shipping `first-contentful-paint`.

/Platform coverage/: All Platforms

/Preference/: N/A

/DevTools bug/: N/A

/Other browsers/: Chrome has it shipped since version 60 (Chrome ships
both `first-paint` and `first-contentful-paint`). Safari also has it
implemented https://bugs.webkit.org/show_bug.cgi?id=78011
(`first-contentful-paint` only), however I am not sure if it's still
behind the experimental flag.

/web-platform-tests/:
https://github.com/web-platform-tests/wpt/tree/master/paint-timing/fcp-only

Thanks,

Sean