Modify languageSettingsPrivate IDL and flesh out stub implementation

extensions/browser/extension_event_histogram_value.h

Index: extensions/browser/extension_event_histogram_value.h
diff --git a/extensions/browser/extension_event_histogram_value.h b/extensions/browser/extension_event_histogram_value.h
index 92054c0919a6258e56120ecff0928a630315c446..aecede9ff58df02f420d6c625da4701564ef783a 100644
--- a/extensions/browser/extension_event_histogram_value.h
+++ b/extensions/browser/extension_event_histogram_value.h
@@ -364,6 +364,10 @@ enum HistogramValue {
   TYPES_PRIVATE_CHROME_DIRECT_SETTING_ON_CHANGE,
   WEB_VIEW_INTERNAL_ON_MESSAGE,  // TODO(kalman): where are the rest of the
                                  // webview events?
+  LANGUAGE_SETTINGS_PRIVATE_ON_INPUT_METHOD_ADDED,
+  LANGUAGE_SETTINGS_PRIVATE_ON_INPUT_METHOD_REMOVED,
+  LANGUAGE_SETTINGS_PRIVATE_ON_SPELLCHECK_DICTIONARIES_CHANGED,
+  LANGUAGE_SETTINGS_PRIVATE_ON_CUSTOM_DICTIONARY_CHANGED,
   // Last entry: Add new entries above, then run:
   // python tools/metrics/histograms/update_extension_histograms.py
   ENUM_BOUNDARY