Add note to ui/gfx/vector_icons/BUILD to reduce developer confusion as

ui/gfx/vector_icons/BUILD.gn

Index: ui/gfx/vector_icons/BUILD.gn
diff --git a/ui/gfx/vector_icons/BUILD.gn b/ui/gfx/vector_icons/BUILD.gn
index 1bd790f19c190a480ec95ddc1486270fe4c223c5..7dad812c8c9ad4ae65456595ca9b0ee739822bd2 100644
--- a/ui/gfx/vector_icons/BUILD.gn
+++ b/ui/gfx/vector_icons/BUILD.gn
@@ -4,6 +4,9 @@
 
 import("//build/config/chrome_build.gni")
 
+# NB: This is a deprecated vector icon target that should be removed. Add vector
+# icons to a more specific target, or ui/vector_icons if ui/ is the right place
+# for them. TODO(estade): remove this target.
 action("aggregate_vector_icons") {
   visibility = [ ":*" ]