fix(thread-page-design): show warning when the experiment is enabled

The warning didn't show correctly because we returned |true| instead of
|{result: true}|.

Change-Id: I96a76bce583f0c178163c678ca533b0cce7b4c6a
diff --git a/src/contentScripts/communityConsole/threadPageDesignWarning.js b/src/contentScripts/communityConsole/threadPageDesignWarning.js
index f58112e..155d916 100644
--- a/src/contentScripts/communityConsole/threadPageDesignWarning.js
+++ b/src/contentScripts/communityConsole/threadPageDesignWarning.js
@@ -126,7 +126,7 @@
           // If the global SMEI experiment is enabled, all threads use nested
           // replies, so we'll skip the per-thread check and always show the
           // warning banner.
-          if (this.isExperimentEnabled) return Promise.resolve(true);
+          if (this.isExperimentEnabled) return Promise.resolve({result: true});
 
           let currentThread = parseUrl(location.href);
           if (currentThread === false)