Add a Finch feature and runtime flag for multidevice.

BUG=718617

Review-Url: https://codereview.chromium.org/2964153002
Cr-Commit-Position: refs/heads/master@{#483827}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 805f114..dfd1d0a 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -1442,6 +1442,9 @@
     {"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",