Replace --add-to-shelf flag with kAppBanners feature.

chrome/browser/about_flags.cc

Index: chrome/browser/about_flags.cc
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 771ba1d93cb6b8db7db5af4bbedfb5d910904f12..b735501a11cbf247e182e9bbcd2e84406ad232c2 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -1985,10 +1985,11 @@ const FeatureEntry kFeatureEntries[] = {
     {"allow-insecure-localhost", flag_descriptions::kAllowInsecureLocalhostName,
      flag_descriptions::kAllowInsecureLocalhostDescription, kOsAll,
      SINGLE_VALUE_TYPE(switches::kAllowInsecureLocalhost)},
-    {"enable-add-to-shelf", flag_descriptions::kAddToShelfName,
-     flag_descriptions::kAddToShelfDescription, kOsDesktop,
-     ENABLE_DISABLE_VALUE_TYPE(switches::kEnableAddToShelf,
-                               switches::kDisableAddToShelf)},
+#if !defined(OS_ANDROID)

[Matt Giuca, 2017/06/22 01:40:38]

Why !defined(OS_ANDROID)?

You already have kOsDesktop. Is the compile-time check necessary?

(The difference is, without the compile-time check, this will show up in
Android's flags as "not available on this platform" which is preferable to it
being confusingly missing.)

[benwells, 2017/06/22 05:57:48]

As discussed it's needed as the feature doesn't exist on Android, so otherwise
the FEATURE_VALUE_TYPE bit won't compile.

+    {"enable-app-banners", flag_descriptions::kAppBannersName,
+     flag_descriptions::kAppBannersDescription, kOsDesktop,
+     FEATURE_VALUE_TYPE(features::kAppBanners)},
+#endif
     {"bypass-app-banner-engagement-checks",
      flag_descriptions::kBypassAppBannerEngagementChecksName,
      flag_descriptions::kBypassAppBannerEngagementChecksDescription, kOsAll,