Add a Finch feature and runtime flag for multidevice.

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 7bc31c6e9011cd1ac0f9010f244131ce6e9d839f..6e7d90313ae9b67876429750f7f8ff73aeb85042 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -1442,6 +1442,9 @@ const FeatureEntry kFeatureEntries[] = {
     {"instant-tethering", flag_descriptions::kTetherName,
      flag_descriptions::kTetherDescription, kOsCrOS,
      FEATURE_VALUE_TYPE(features::kInstantTethering)},
+    {"multidevice", flag_descriptions::kMultideviceName,
+     flag_descriptions::kMultideviceDescription, kOsCrOS,
+     FEATURE_VALUE_TYPE(features::kMultidevice)},
 #endif  // OS_CHROMEOS
     {
         "disable-accelerated-video-decode",