MD WebUI: Update docs for custom iconsets.

ui/webui/resources/cr_elements/icons.html

Index: ui/webui/resources/cr_elements/icons.html
diff --git a/ui/webui/resources/cr_elements/icons.html b/ui/webui/resources/cr_elements/icons.html
index f0e62a9b849c380b39ad1753ce4d9ff415ee0362..e0ea0ea29a76d498add063fd3469c357296d5b18 100644
--- a/ui/webui/resources/cr_elements/icons.html
+++ b/ui/webui/resources/cr_elements/icons.html
@@ -9,9 +9,8 @@ Do not add rarely used icons here; place those in your application.
   <svg>
     <defs>
       <!--
-      These icons are from iron-icons in third_party/polymer, but were curated
-      here via the following URL and then sorted:
-      https://poly-icon.appspot.com/?name=cr&size=24&icon=icons:arrow-drop-down&icon=icons:cancel&icon=icons:check&icon=icons:chevron-right&icon=icons:clear&icon=icons:expand-less&icon=icons:expand-more&icon=icons:extension&icon=icons:file-download&icon=icons:fullscreen&icon=icons:more-vert&icon=icons:print&icon=icons:search&icon=icons:settings&icon=icons:star&icon=icons:star-border&icon=social:domain&icon=social:group&icon=social:person
+      These icons are copied from Polymer's iron-icons and kept in sorted order.
+      See http://goo.gl/Y1OdAq for instructions on adding additional icons.
       -->
       <g id="arrow-drop-down"><path d="M7 10l5 5 5-5z"></path></g>
       <g id="cancel"><path d="M12 2C6.47 2 2 6.47 2 12s4.47 10 10 10 10-4.47 10-10S17.53 2 12 2zm5 13.59L15.59 17 12 13.41 8.41 17 7 15.59 10.59 12 7 8.41 8.41 7 12 10.59 15.59 7 17 8.41 13.41 12 17 15.59z"></path></g>