[IntentHandling] Add flag for app service external protocol handling.

This CL adds a feature flag for using the App Service to provide data
for external protocol dialog on Chrome OS.

BUG=853604

Change-Id: I4ee2273fc78599c2ec77ec5844f2903a24c187dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2415917
Reviewed-by: Dominick Ng <[email protected]>
Commit-Queue: Maggie Cai <[email protected]>
Cr-Commit-Position: refs/heads/master@{#807883}
diff --git a/chrome/browser/flag_descriptions.cc b/chrome/browser/flag_descriptions.cc
index 7a9223a..84452af 100644
--- a/chrome/browser/flag_descriptions.cc
+++ b/chrome/browser/flag_descriptions.cc
@@ -3580,6 +3580,10 @@
     "Shows settings to enable/disable scroll acceleration and to adjust the "
     "sensitivity for scrolling.";
 
+const char kAppServiceExternalProtocolName[] = "App Service External Protocol";
+const char kAppServiceExternalProtocolDescription[] =
+    "Use the App Service to provide data for external protocol dialog.";
+
 const char kAppServiceIntentHandlingName[] = "App Service Intent Handling";
 const char kAppServiceIntentHandlingDescription[] =
     "Use the App Service to provide data for intent handling.";