Add about:flags control for toggling sync server sandbox

chrome/browser/about_flags.cc

Index: chrome/browser/about_flags.cc
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index e63b0fe6ce8ebdc358347c200e61219f1a3039a3..781a4e53730b2a558386cc2263a848b7998f19a3 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -2190,6 +2190,15 @@ const Experiment kExperiments[] = {
     kOsAndroid,
     SINGLE_VALUE_TYPE(switches::kBypassAppBannerEngagementChecks)
   },
+  {
+    "use-sync-sandbox",
+    IDS_FLAGS_SYNC_SANDBOX_NAME,
+    IDS_FLAGS_SYNC_SANDBOX_DESCRIPTION,
+    kOsAll,
+    SINGLE_VALUE_TYPE_AND_VALUE(
+        switches::kSyncServiceUrl,
+        "https://chrome-sync.sandbox.google.com/chrome-sync/alpha")
+  },
 
   // NOTE: Adding new command-line switches requires adding corresponding
   // entries to enum "LoginCustomFlags" in histograms.xml. See note in