cryptotoken: add a UI flag for the U2FSecurityKeyAPI base::Feature

The U2fSecurityKeyAPI feature controls whether the deprecated U2F
Security Key API is enabled. But features are less than easy to enable
for users, and cannot be controlled via enterprise policy. But we want
both of those. Making the feature a UI flag is the solution.

Bug: 1224886
Change-Id: Ia2704a3bca03f304f807f0eba3959b5ca913d6ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3169469
Commit-Queue: Martin Kreichgauer <[email protected]>
Reviewed-by: Adam Langley <[email protected]>
Cr-Commit-Position: refs/heads/main@{#922757}
diff --git a/chrome/browser/flag_descriptions.cc b/chrome/browser/flag_descriptions.cc
index 866a2be..7a68047 100644
--- a/chrome/browser/flag_descriptions.cc
+++ b/chrome/browser/flag_descriptions.cc
@@ -183,6 +183,13 @@
     "a blob and the verification process of the signed exchange is skipped for "
     "the succeeding navigation.";
 
+const char kU2FSecurityKeyAPIName[] = "Enable the U2F Security Key API";
+const char kU2FSecurityKeyAPIDescription[] =
+    "Enable the legacy U2F Security Key API (CryptoToken). The U2F Security "
+    "Key API has been deprecated and will be removed soon. For more "
+    "information, refer to the deprecation announcement at "
+    "https://groups.google.com/a/chromium.org/g/blink-dev/c/xHC3AtU_65A";
+
 const char kUseLookalikesForNavigationSuggestionsName[] =
     "Use lookalike URL suggestions for navigation suggestions";
 const char kUseLookalikesForNavigationSuggestionsDescription[] =