Enable SummaryListItem flag by default

* Make the default 'display' property value 'list-item' for the first
  <summary> child of a <details>

* Remove ::-webkit-details-marker pseudo element, and warn if it is
  used.

* Developers can use ::marker pseudo element instead. However, many CSS
  properties don't work.
  https://drafts.csswg.org/css-lists-3/#marker-properties

I2S: https://t.co/g94jiQq604

Bug: 590014
Change-Id: I8dd14ebffeacee8b7f8a04f7b68f1980a4063d85
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2586164Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835920}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -1923,7 +1923,7 @@
     },
     {
       name: "SummaryListItem",
-      status: "test",
+      status: "stable",
     },
     {
       name: "SurfaceEmbeddingFeatures",