Add chrome://flags entry for BlockTabUnders on Android

The UI is not implemented yet on desktop.

Bug: 661629
Change-Id: Iee0b2134d02eb63325b20376c38500a38d49fb9a
Reviewed-on: https://chromium-review.googlesource.com/701474
Reviewed-by: Avi Drissman <[email protected]>
Commit-Queue: Charlie Harrison <[email protected]>
Cr-Commit-Position: refs/heads/master@{#506616}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index c24fc964..7e27922 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -69,6 +69,9 @@
 extern const char kBleAdvertisingInExtensionsName[];
 extern const char kBleAdvertisingInExtensionsDescription[];
 
+extern const char kBlockTabUndersName[];
+extern const char kBlockTabUndersDescription[];
+
 extern const char kBrowserSideNavigationName[];
 extern const char kBrowserSideNavigationDescription[];