Follow up to 'Add enabledExperiments to soy template'

Issue:
At present state adding experimental flags results in the following
line being added to the '<script>' section of header:

  window.ENABLED_EXPERIMENTS = JSON.parse(["flag"])

and JSON.parse fails with the following error:

  Uncaught SyntaxError: Unexpected token l in JSON at position 1
    at JSON.parse (<anonymous>)
    at <anonymous>:1:6

Solution:
Turn the sanitized object toString() so that it results in JSON-parsable
string:

  window.ENABLED_EXPERIMENTS = JSON.parse('\x5b\x22flag\x22\x5d')

Change-Id: Ida7e8faa6679880a2204489fbf604fee27ae2cd0

java/com/google/gerrit/httpd/raw/IndexHtmlUtil.java

@@ -125,7 +125,7 @@ public class IndexHtmlUtil {
 
     Set<String> enabledExperiments = experimentData(urlParameterMap);
     if (!enabledExperiments.isEmpty()) {
-      data.put("enabledExperiments", serializeObject(GSON, enabledExperiments));
+      data.put("enabledExperiments", serializeObject(GSON, enabledExperiments).toString());
     }
     return data.build();
   }