Enabled Customized Built-in elements by default.

Approved intent-to-ship:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/S3naD9vIcM4/7tKyfvi0AgAJ

Bug: 648828
Change-Id: Ia4e1d43170f2f298f1e55bad6bc4173556cd2ca9
Reviewed-on: https://chromium-review.googlesource.com/965641Reviewed-by: Yoichi Osato <yoichio@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543606}

third_party/WebKit/Source/platform/runtime_enabled_features.json5

@@ -341,7 +341,7 @@
     },
     {
       name: "CustomElementsBuiltin",
-      status: "experimental",
+      status: "stable",
     },
     {
       name: "CustomUserTiming",