Add blink feature flag for Off-Thread PaintWorklet

Bug: 884711
Change-Id: Ice9b8c2d260a98f5612884a19abca85d200e5ee4
Reviewed-on: https://chromium-review.googlesource.com/1240354Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593577}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -844,6 +844,9 @@
       name: "NullableDocumentDomain",
       status: "experimental",
     },
+    {
+      name: "OffMainThreadCSSPaint",
+    },
     {
       name: "OffMainThreadWebSocket",
     },