Hotword Audio Verification app: control the hotword settings

Adds a function to the hotword private API to allow the Hotword Audio
Verification app to set the always-on-hotword setting.

Gives control over the always-on-hotword pref to the app from the settings page.

This cl is dependent on https://codereview.chromium.org/528113003/

BUG=390086

Review URL: https://codereview.chromium.org/559303002

Cr-Commit-Position: refs/heads/master@{#296661}
diff --git a/extensions/browser/extension_function_histogram_value.h b/extensions/browser/extension_function_histogram_value.h
index fab2dc18..28e3c75 100644
--- a/extensions/browser/extension_function_histogram_value.h
+++ b/extensions/browser/extension_function_histogram_value.h
@@ -956,6 +956,7 @@
   WEBVIEWINTERNAL_SETALLOWTRANSPARENCY,
   EASYUNLOCKPRIVATE_GETUSERINFO,
   HOTWORDPRIVATE_GETLAUNCHSTATE,
+  HOTWORDPRIVATE_SETHOTWORDALWAYSONSEARCHENABLED,
   // Last entry: Add new entries above and ensure to update
   // tools/metrics/histograms/histograms.xml.
   ENUM_BOUNDARY