Add a "kill switch" Finch flag for backdrop-filter

This enables a user-visible flag for backdrop-filter, so that
Finch can be used to disable it in the field if needed.

Change-Id: Ieb6388b1f2f4f3daebc81d2775c816a423f5642b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1658992
Reviewed-by: Kentaro Hara <[email protected]>
Commit-Queue: Mason Freed <[email protected]>
Cr-Commit-Position: refs/heads/master@{#669313}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 5c160602..d0bd56cb 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -399,6 +399,9 @@
 extern const char kEnableBlinkGenPropertyTreesName[];
 extern const char kEnableBlinkGenPropertyTreesDescription[];
 
+extern const char kEnableCSSBackdropFilterName[];
+extern const char kEnableCSSBackdropFilterDescription[];
+
 extern const char kEnableDisplayLockingName[];
 extern const char kEnableDisplayLockingDescription[];