custom-elements: Promote CustomElementsBuiltin flag to "experimental"

Bug: 648828
Change-Id: If19afe0ad2aa29eb5a042306f260d5eac77e0f0e
Reviewed-on: https://chromium-review.googlesource.com/895423Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533250}

third_party/WebKit/Source/platform/runtime_enabled_features.json5

@@ -312,7 +312,7 @@
     },
     {
       name: "CustomElementsBuiltin",
-      status: "test",
+      status: "experimental",
     },
     {
       name: "CustomUserTiming",