arc: Add Custom Tabs experiment

BUG=b:129835848
TEST=build

Change-Id: Ie074e6c67f0134d49098557071480663176b9968
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1549302
Commit-Queue: Ryo Hashimoto <[email protected]>
Reviewed-by: Yusuke Sato <[email protected]>
Reviewed-by: Scott Violet <[email protected]>
Cr-Commit-Position: refs/heads/master@{#648623}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index b33d9be..fe0330b5 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -1777,6 +1777,9 @@
 extern const char kArcCupsApiName[];
 extern const char kArcCupsApiDescription[];
 
+extern const char kArcCustomTabsExperimentName[];
+extern const char kArcCustomTabsExperimentDescription[];
+
 extern const char kArcDocumentsProviderName[];
 extern const char kArcDocumentsProviderDescription[];