[NTP] Enable MD icons when custom backgrounds is enabled

The Material Design icons for Most Visited should be shown when custom
backgrounds is enabled. Also moved kNTPIcons to ntp_features.h since it
is no longer used in the ntp_tiles component.

See go/chrome-ntp-birthday-flags

Change-Id: If8e5d9cad0f09ace08aa8736df9b69b717fc4b34
Reviewed-on: https://chromium-review.googlesource.com/1191815
Commit-Queue: Kristi Park <[email protected]>
Reviewed-by: Scott Violet <[email protected]>
Reviewed-by: Marc Treib <[email protected]>
Reviewed-by: Mathieu Perreault <[email protected]>
Reviewed-by: Pavol Marko <[email protected]>
Cr-Commit-Position: refs/heads/master@{#586814}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index a35b486..581fd78 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -4096,7 +4096,7 @@
 
     {"ntp-icons", flag_descriptions::kNtpIconsName,
      flag_descriptions::kNtpIconsDescription, kOsDesktop,
-     FEATURE_VALUE_TYPE(ntp_tiles::kNtpIcons)},
+     FEATURE_VALUE_TYPE(features::kNtpIcons)},
 
     {"ntp-ui-md", flag_descriptions::kNtpUIMdName,
      flag_descriptions::kNtpUIMdDescription, kOsDesktop,