Support Paint Timing Mixin

[jdapena]

Is your feature request related to a problem? Please describe.
The performance entries related to painting are proggressively adding support in the specification for the Paint Timing Mixin. Container Timing also reports paints, so it should also support it in alignment with other specifications.

Describe the solution you'd like
The explainer should add a reference to the Paint Timing Mixin, in a similar way to what is done in other elements (at least making it explicit the entry includes PaintTimingMixin).

Describe alternatives you've considered
Alternative zero: doing nothing. It has the problem that it potentially creates an inconsistency with other methods already supporting it.

Additional context
Chromium supports the Paint Timing Mixin, under a feature flag. The native implementation there will be under the same feature flag.