Add flag-protected security key UI under Passwords.

This change adds a hidden-by-default entry in the passwords settings
UI on Android that triggers the Phone as a Security Key prototype code.

The most important thing to verify about this change is that it doesn't
have any effect unless the user manually sets the flag: the UI is
intended for experimentation and for UX-team to consider at this point.

BUG: 1002262
Change-Id: I17d1077002b53198bc1d2422e28d13c6611dfc5b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2079614
Commit-Queue: Adam Langley <[email protected]>
Auto-Submit: Adam Langley <[email protected]>
Reviewed-by: Friedrich [CET] <[email protected]>
Cr-Commit-Position: refs/heads/master@{#748453}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 34da09d..6ab1389 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -3328,12 +3328,10 @@
          kMarkHttpAsFeatureVariations,
          "HTTPReallyBadFinal")},
 
-#if !defined(OS_ANDROID)
     {"enable-web-authentication-cable-v2-support",
      flag_descriptions::kEnableWebAuthenticationCableV2SupportName,
      flag_descriptions::kEnableWebAuthenticationCableV2SupportDescription,
-     kOsDesktop, FEATURE_VALUE_TYPE(device::kWebAuthPhoneSupport)},
-#endif  // !defined(OS_ANDROID)
+     kOsDesktop | kOsAndroid, FEATURE_VALUE_TYPE(device::kWebAuthPhoneSupport)},
 
     {"use-preferred-interval-for-video",
      flag_descriptions::kUsePreferredIntervalForVideoName,