Allow overriding variation parameter via chrome://flags.

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 85a46e06e4d42ce154f059336b6daa7d9abc72ee..221928dae93ac7502e99ebaf4694fc32de119a71 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -2012,6 +2012,12 @@ void ConvertFlagsToSwitches(flags_ui::FlagsStorage* flags_storage,
       switches::kDisableFeatures);
 }
 
+void RegisterAllFeatureVariationParameters(
+    flags_ui::FlagsStorage* flags_storage) {
+  FlagsStateSingleton::GetFlagsState()->RegisterAllFeatureVariationParameters(
+      flags_storage);
+}
+
 bool AreSwitchesIdenticalToCurrentCommandLine(
     const base::CommandLine& new_cmdline,
     const base::CommandLine& active_cmdline,