Add `privacy.network.webRTCMultipleRoutesEnabled` to extensions API.

chrome/test/data/extensions/api_test/preference/standard/test.js

Index: chrome/test/data/extensions/api_test/preference/standard/test.js
diff --git a/chrome/test/data/extensions/api_test/preference/standard/test.js b/chrome/test/data/extensions/api_test/preference/standard/test.js
index 972c2b98af877e0be94b622fb2854af621a5228d..905bdbf3b3230af4097bd48ba72500432e1aebe0 100644
--- a/chrome/test/data/extensions/api_test/preference/standard/test.js
+++ b/chrome/test/data/extensions/api_test/preference/standard/test.js
@@ -9,7 +9,8 @@ var preferences_to_test = [
   {
     root: chrome.privacy.network,
     preferences: [
-      'networkPredictionEnabled'
+      'networkPredictionEnabled',
+      'webRTCMultipleRoutesEnabled'
     ]
   },
   {
@@ -37,6 +38,13 @@ var preferences_to_test = [
   },
 ];
 
+// Some preferences are only present on certain platforms or are hidden
+// behind flags and might not be present when this test runs.
+var possibly_missing_preferences = new Set([
+  'protectedContentEnabled',    // Windows/ChromeOS only
+  'webRTCMultipleRoutesEnabled' // requires ENABLE_WEBRTC=1
+]);
+
 function expect(expected, message) {
   return chrome.test.callbackPass(function(value) {
     chrome.test.assertEq(expected, value, message);
@@ -51,18 +59,14 @@ function expectFalse(pref) {
 }
 
 function prefGetter(pref) {
-  if (pref === 'protectedContentEnabled' && !this[pref]) {
-    // `protectedContentEnabled` is Windows/ChromeOS only, so it might not exist
-    // when this test runs, and that's pretty much OK.
+  if (possibly_missing_preferences.has(pref) && !this[pref]) {
     return true;
   }
   this[pref].get({}, expectFalse(pref));
 }
 
 function prefSetter(pref) {
-  if (pref === 'protectedContentEnabled' && !this[pref]) {
-    // `protectedContentEnabled` is Windows/ChromeOS only, so it might not exist
-    // when this test runs, and that's pretty much OK.
+  if (possibly_missing_preferences.has(pref) && !this[pref]) {
     return true;
   }
   this[pref].set({value: true}, chrome.test.callbackPass());