Add Feature and about:flag for root scroller

This will be used to allow a kill-switch for the feature post-launch as
well as help debugging.

Bug: 903263
Change-Id: Icaf0b9ea4f28f9abddfa67f88fc6759aca812ec3
Reviewed-on: https://chromium-review.googlesource.com/c/1349476
Reviewed-by: Avi Drissman <[email protected]>
Reviewed-by: Chris Harrelson <[email protected]>
Commit-Queue: David Bokan <[email protected]>
Cr-Commit-Position: refs/heads/master@{#611404}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 935218c..1e1e989 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -374,6 +374,9 @@
 extern const char kEnableHeavyPageCappingName[];
 extern const char kEnableHeavyPageCappingDescription[];
 
+extern const char kEnableImplicitRootScrollerName[];
+extern const char kEnableImplicitRootScrollerDescription[];
+
 extern const char kEnablePreviewsAndroidOmniboxUIName[];
 extern const char kEnablePreviewsAndroidOmniboxUIDescription[];