Just to clarify - these should be web platform tests, correct?

I don't think this is super high priority. npm@ - sticking this on your backlog, but leaving it as a P3.

yeah web-platform-tests

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/12c3e9289a6427903f13c9aeccf6b052d4eb4594

commit 12c3e9289a6427903f13c9aeccf6b052d4eb4594
Author: Nicolas Pena <npm@chromium.org>
Date: Thu Nov 09 19:59:05 2017

Add paint-timing test for child iframe

This CL ensures that a child iframe does not report paint timing entries
when only the parent does all of the painting.

Bug:  chromium:736114 
Change-Id: Idcba2bb9e93eeb7168e604840df4497df4073472
Reviewed-on: https://chromium-review.googlesource.com/755857
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515253}
[add] https://crrev.com/12c3e9289a6427903f13c9aeccf6b052d4eb4594/third_party/WebKit/LayoutTests/external/wpt/paint-timing/first-image-child.html
[add] https://crrev.com/12c3e9289a6427903f13c9aeccf6b052d4eb4594/third_party/WebKit/LayoutTests/external/wpt/paint-timing/resources/subframe-sending-paint.html

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/32522b611fe1b8c5dfb2e8111baca350115dc19d

commit 32522b611fe1b8c5dfb2e8111baca350115dc19d
Author: Nicolas Pena <npm@chromium.org>
Date: Tue Nov 14 17:18:53 2017

Add paint-timing test when child iframe paints

This CL adds a test with a frame having a child iframe that renders an
image. the parent should only get a first-paint entry even after the
child has rendered the image.

An upstream spec issue had been filed to clarify:
https://github.com/w3c/paint-timing/issues/22

Bug:  736114 
Change-Id: Idc0910a7acc8f8af3bf9634a7e6827805fe8d370
Reviewed-on: https://chromium-review.googlesource.com/766393
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516339}
[modify] https://crrev.com/32522b611fe1b8c5dfb2e8111baca350115dc19d/third_party/WebKit/LayoutTests/external/wpt/lint.whitelist
[add] https://crrev.com/32522b611fe1b8c5dfb2e8111baca350115dc19d/third_party/WebKit/LayoutTests/external/wpt/paint-timing/child-painting-first-image.html
[add] https://crrev.com/32522b611fe1b8c5dfb2e8111baca350115dc19d/third_party/WebKit/LayoutTests/external/wpt/paint-timing/resources/subframe-painting.html
[modify] https://crrev.com/32522b611fe1b8c5dfb2e8111baca350115dc19d/third_party/WebKit/LayoutTests/external/wpt/paint-timing/resources/subframe-sending-paint.html

The tests added cover what child sees when parent paints and what parent sees when child paints. Is there any case missing?

The tests are based on image but I'm not sure if there's value in adding analogous tests based on SVG, background color etc. Is there?

Oh I guess siblings is of interest here too. So that should probably be added.

I agree there's no value in covering all cases with child frames.

We should test siblings though.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d5eff2fa70bd01611031a529129e12c970629614

commit d5eff2fa70bd01611031a529129e12c970629614
Author: Nicolas Pena <npm@chromium.org>
Date: Tue Nov 28 19:24:55 2017

Add paint-timing test for sibling frames

This CL adds a test to make sure that there are no paint entries when a
sibling frame paints an image.

Bug:  736114 
Change-Id: Ie65dc1f3aea6d67f690164ba9007950eba7a5c6d
Reviewed-on: https://chromium-review.googlesource.com/769929
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519790}
[modify] https://crrev.com/d5eff2fa70bd01611031a529129e12c970629614/third_party/WebKit/LayoutTests/external/wpt/paint-timing/child-painting-first-image.html
[add] https://crrev.com/d5eff2fa70bd01611031a529129e12c970629614/third_party/WebKit/LayoutTests/external/wpt/paint-timing/sibling-painting-first-image.html