The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cbe3bb5ff15b5ce1b6b1392fdc786087eaedb47a

commit cbe3bb5ff15b5ce1b6b1392fdc786087eaedb47a
Author: Fergal Daly <fergal@chromium.org>
Date: Thu Jan 17 22:50:43 2019

[CSS] Make ::part(foo)::placeholder work.

This copies the ::slotted::placeholder approach and adds a
StyleResolver::MatchPseudoPartRulesForUAHost.

This also adds a reftest due to the fact that getComputedStyle does
not work with "::placeholder" -  https://crbug.com/922431 . It leaves the
old test in place because other browsers may support it and the hopefully
blink will in the future.

Bug:  922431 
Change-Id: Iec4d9b279cf1f82dbdcbcc69dcac873678943882
Reviewed-on: https://chromium-review.googlesource.com/c/1415066
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623885}
[modify] https://crrev.com/cbe3bb5ff15b5ce1b6b1392fdc786087eaedb47a/third_party/blink/renderer/core/css/css_selector.cc
[modify] https://crrev.com/cbe3bb5ff15b5ce1b6b1392fdc786087eaedb47a/third_party/blink/renderer/core/css/css_selector.h
[modify] https://crrev.com/cbe3bb5ff15b5ce1b6b1392fdc786087eaedb47a/third_party/blink/renderer/core/css/resolver/style_resolver.cc
[modify] https://crrev.com/cbe3bb5ff15b5ce1b6b1392fdc786087eaedb47a/third_party/blink/renderer/core/css/resolver/style_resolver.h
[modify] https://crrev.com/cbe3bb5ff15b5ce1b6b1392fdc786087eaedb47a/third_party/blink/renderer/core/css/rule_set.cc
[add] https://crrev.com/cbe3bb5ff15b5ce1b6b1392fdc786087eaedb47a/third_party/blink/web_tests/external/wpt/css/css-shadow-parts/interaction-with-placeholder-ref.html
[add] https://crrev.com/cbe3bb5ff15b5ce1b6b1392fdc786087eaedb47a/third_party/blink/web_tests/external/wpt/css/css-shadow-parts/interaction-with-placeholder.html