Download Bar: Initial feature flag commit

Add a placeholder WebUI download shelf web view implementation.
To enable: --enable-features=WebUIDownloadShelf

Bug: 1182529
Change-Id: I15f83e50c7e62bfed8c88e9015376d06cb29c41c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2716514
Commit-Queue: Roman Arora <[email protected]>
Reviewed-by: Keren Zhu <[email protected]>
Reviewed-by: Yuheng Huang <[email protected]>
Reviewed-by: Peter Kasting <[email protected]>
Reviewed-by: Robert Liao <[email protected]>
Reviewed-by: Thomas Lukaszewicz <[email protected]>
Cr-Commit-Position: refs/heads/master@{#858347}
diff --git a/chrome/browser/ui/ui_features.h b/chrome/browser/ui/ui_features.h
index f6e61bc6..167abf5 100644
--- a/chrome/browser/ui/ui_features.h
+++ b/chrome/browser/ui/ui_features.h
@@ -106,6 +106,8 @@
 
 extern const base::Feature kWebUIBubblePerProfilePersistence;
 
+extern const base::Feature kWebUIDownloadShelf;
+
 extern const base::Feature kWebUITabStrip;
 
 extern const base::Feature kWebUIFeedback;