Enable DeclarativeShadowDOM feature for "experimental"

With this CL, the Enable Experimental Web Platform Features flag
will enable declarative Shadow DOM. This makes it easier to test.

Bug: 1042130
Change-Id: Ib18a9f39e9c3cb3e4a1fb35af95ba469360ab334
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2064465
Commit-Queue: Philip Rogers <pdr@chromium.org>
Auto-Submit: Mason Freed <masonfreed@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742814}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -579,7 +579,7 @@
     },
     {
       name: "DeclarativeShadowDOM",
-      status: "test",
+      status: "experimental",
     },
     {
       name: "DecodeJpeg420ImagesToYUV",