RELNOTES[NEW]: Provide goog.labs.userAgent.USE_CLIENT_HINTS_OVERRIDE …

…as a runtime-override mechanism for overriding USE_CLIENT_HINTS in production builds.

PiperOrigin-RevId: 398765743
Change-Id: I48aec9a4b0e70835867d6344813fcb4653cf8f95

closure/goog/labs/useragent/useragent.js

@@ -10,10 +10,30 @@
 
 goog.module('goog.labs.userAgent');
 
+/**
+ * @define {string} Optional runtime override for the USE_CLIENT_HINTS flag.
+ * If this is set (for example, to 'foo.bar') then any value of USE_CLIENT_HINTS
+ * will be overridden by `globalThis.foo.bar` if it is non-null.
+ * This flag will be removed in December 2021.
+ */
+const USE_CLIENT_HINTS_OVERRIDE =
+    goog.define('goog.labs.userAgent.USE_CLIENT_HINTS_OVERRIDE', '');
+
 /**
  * @define {boolean} If true, use navigator.userAgentData
  * TODO(user) Flip flag in 2021/12.
  */
 const USE_CLIENT_HINTS =
     goog.define('goog.labs.userAgent.USE_CLIENT_HINTS', false);
-exports.USE_CLIENT_HINTS = USE_CLIENT_HINTS;
+
+// TODO(user): Replace the IIFE with a simple null-coalescing operator.
+// NOTE: This can't be done with a helper function, or else we risk an inlining
+// back-off causing a huge code size regression if a non-inlined helper function
+// prevents the optimizer from detecting the (possibly large) dead code paths.
+/** @const {boolean} */
+exports.USE_CLIENT_HINTS = (() => {
+  const override = USE_CLIENT_HINTS_OVERRIDE ?
+         goog.getObjectByName(USE_CLIENT_HINTS_OVERRIDE) :
+         null;
+  return override != null ? override : USE_CLIENT_HINTS;
+})();