[css-properties-values-api] Enable CSS.registerProperty by default.

Note that <image>, <transform-list> and <transform-function> syntaxes
aren't shipping at this time, and remain enabled for testing only.

Intent to Ship: https://groups.google.com/a/chromium.org/d/msg/blink-dev/QjcyhPZ_sNI/SuB-GuNPDAAJ

Bug: 641877
Change-Id: I548db4896c355ab61c0b3a98bfa5119c2fd896f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1623945
Auto-Submit: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662573}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -392,7 +392,7 @@
     },
     {
       name: "CSSVariables2",
-      status: "experimental",
+      status: "stable",
     },
     // Support for registered custom properties with <image> syntax.
     {

third_party/blink/web_tests/virtual/stable/webexposed/global-interface-listing-expected.txt

@@ -411,6 +411,7 @@ interface CSS
     static method pt
     static method px
     static method rad
+    static method registerProperty
     static method rem
     static method s
     static method supports