Adopt firstMeaningfulPaint trace event

[paulirish]

• category: blink.user_timing
• name: firstMeaningfulPaint

According to new data (https://docs.google.com/document/d/16ItLkzm0IX2RKoAlztqrAmv30L9mJcDeYLJuGsH6QUo/edit#), the trace event implementation has better accuracy than the trace-based one. (both the LH impl and the catapult one)

This event is in m54, so we'll have some time before that's in stable. I propose we use our calculation as fallback until then.

Once we're ready to remove we can nuke the disabled-by-default-blink.debug.layout category as well.

[irori]

Note that blink.user_timing:firstMeaningfulPaint is not logged until network activity of the page is stabilized (currently 2 secs of no network activity, but it's subject to change).

Instead, catapult uses loading:firstMeaningfulPaintCandidate trace event which is logged every time layout significance marks a record. The timestamp of the last candidate is the time of first-meaningful-paint.

[paulirish]

Super useful, thanks.

The 2sec-of-no-network-activity heuristic is probably something we should match in #627 as well.

[wardpeet]

@brendankenny @paulirish Mind if I take this one? (remove me if you rather want to do it)