MD WebUI: Update docs for custom iconsets.

chrome/browser/resources/settings/icons.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-iconset-svg/iron-iconset-svg.html">
 
 <iron-iconset-svg name="settings" size="24">
   <svg>
     <defs>
       <!-- Cookie SVG obtained from rolfe@ -->
       <g id="cookie">
         <path d="M15.5 7.5V7c0-.98-.5-1.5-1.5-1.5h-.5c-.276 0-.5-.224-.5-.5V3c0-.98-1-1-1-1C6.3 2 1.712 6.77 2.014 12.54c.265 5.046 4.4 9.18 9.448 9.446C17.23 22.288 22 17.7 22 12v-1c0-.553-.447-1-1-1h-1.998c-.277 0-.502-.225-.502-.502V9c0-.938-.48-1.48-1.5-1.5h-1.5zm-9.706 4.972c-1.057.2-1.966-.71-1.766-1.766.112-.587.592-1.067 1.18-1.178 1.055-.2 1.965.71 1.764 1.765-.11.588-.59 1.068-1.178 1.18zm1.734-5.178c-.2-1.057.71-1.966 1.766-1.766.587.11 1.067.59 1.178 1.178.2 1.057-.708 1.966-1.765 1.766-.587-.11-1.068-.59-1.18-1.178zm3.766 12.178c-1.057.2-1.966-.71-1.766-1.766.112-.587.592-1.067 1.18-1.178 1.056-.2 1.965.71 1.764 1.766-.11.587-.59 1.067-1.178 1.178zM11.5 14c-.828 0-1.5-.67-1.5-1.5s.672-1.5 1.5-1.5c.83 0 1.5.67 1.5 1.5s-.67 1.5-1.5 1.5zm5 2c-.828 0-1.5-.67-1.5-1.5s.672-1.5 1.5-1.5c.83 0 1.5.67 1.5 1.5s-.67 1.5-1.5 1.5z" fill-rule="evenodd"></path>
       </g>
 
       <!-- The Google "G" icon in the Clear Browsing Data dialog. -->
       <g id="googleg">
         <path fill="#4285F4" d="M7.36 4.08c0-.23-.025-.405-.055-.58H4v1.2h1.985c-.05.31-.26.795-.725 1.13v.82h1.06c.66-.605 1.04-1.505 1.04-2.57z"></path>
         <path fill="#34A853" d="M4 7.5c.945 0 1.735-.315 2.315-.845l-1.06-.82c-.3.215-.71.375-1.255.375-.945 0-1.74-.62-2.04-1.48H.875v.845C1.45 6.715 2.63 7.5 4 7.5z"></path>
         <path fill="#FBBC05" d="M1.96 4.725c-.075-.225-.13-.47-.13-.725s.05-.5.13-.725V2.43H.875C.635 2.905.5 3.435.5 4s.135 1.095.375 1.57l1.085-.845z"></path>
         <path fill="#EA4335" d="M4 1.79c.68 0 1.135.29 1.395.54l.95-.915C5.735.845 4.945.5 4 .5 2.63.5 1.45 1.285.875 2.43l1.085.845C2.26 2.415 3.055 1.79 4 1.79z"></path>
         <path fill="none" d="M.5.5h7v7h-7z"></path>
       </g>
 
       <!--
-      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=settings&size=24&icon=av:mic&icon=av:videocam&icon=av:volume-up&icon=av:web&icon=communication:location-on&icon=device:access-time&icon=device:battery-charging-full&icon=device:bluetooth&icon=device:network-wifi&icon=device:wallpaper&icon=hardware:desktop-windows&icon=hardware:keyboard&icon=hardware:laptop-chromebook&icon=hardware:security&icon=icons:accessibility&icon=icons:apps&icon=icons:arrow-back&icon=icons:arrow-drop-up&icon=icons:assignment&icon=icons:build&icon=icons:clear&icon=icons:close&icon=icons:cloud&icon=icons:content-copy&icon=icons:delete&icon=icons:done&icon=icons:error&icon=icons:folder&icon=icons:info&icon=icons:input&icon=icons:language&icon=icons:list&icon=icons:lock&icon=icons:menu&icon=icons:open-in-new&icon=icons:power-settings-new&icon=icons:refresh&icon=icons:restore&icon=icons:settings&icon=icons:supervisor-account&icon=icons:visibility&icon=icons:warning&icon=image:brightness-1&icon=image:camera-alt&icon=image:flip&icon=image:palette&icon=image:photo&icon=image:rotate-right&icon=notification:sim-card-alert&icon=social:notifications&icon=social:people
+      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.
       -->
 <if expr="chromeos">
       <g id="access-time"><path d="M11.99 2C6.47 2 2 6.48 2 12s4.47 10 9.99 10C17.52 22 22 17.52 22 12S17.52 2 11.99 2zM12 20c-4.42 0-8-3.58-8-8s3.58-8 8-8 8 3.58 8 8-3.58 8-8 8zm.5-13H11v6l5.25 3.15.75-1.23-4.5-2.67z"></path></g>
 </if>
       <g id="accessibility"><path d="M12 2c1.1 0 2 .9 2 2s-.9 2-2 2-2-.9-2-2 .9-2 2-2zm9 7h-6v13h-2v-6h-2v6H9V9H3V7h18v2z"></path></g>
       <g id="apps"><path d="M4 8h4V4H4v4zm6 12h4v-4h-4v4zm-6 0h4v-4H4v4zm0-6h4v-4H4v4zm6 0h4v-4h-4v4zm6-10v4h4V4h-4zm-6 4h4V4h-4v4zm6 6h4v-4h-4v4zm0 6h4v-4h-4v4z"></path></g>
       <g id="arrow-back"><path d="M20 11H7.83l5.59-5.59L12 4l-8 8 8 8 1.41-1.41L7.83 13H20v-2z"></path></g>
       <g id="arrow-drop-up"><path d="M7 14l5-5 5 5z"></path></g>
       <g id="assignment"><path d="M19 3h-4.18C14.4 1.84 13.3 1 12 1c-1.3 0-2.4.84-2.82 2H5c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h14c1.1 0 2-.9 2-2V5c0-1.1-.9-2-2-2zm-7 0c.55 0 1 .45 1 1s-.45 1-1 1-1-.45-1-1 .45-1 1-1zm2 14H7v-2h7v2zm3-4H7v-2h10v2zm0-4H7V7h10v2z"></path></g>
 <if expr="chromeos">
@@ skipping 60 matching lines @@
 <if expr="chromeos">
       <g id="wallpaper"><path d="M4 4h7V2H4c-1.1 0-2 .9-2 2v7h2V4zm6 9l-4 5h12l-3-4-2.03 2.71L10 13zm7-4.5c0-.83-.67-1.5-1.5-1.5S14 7.67 14 8.5s.67 1.5 1.5 1.5S17 9.33 17 8.5zM20 2h-7v2h7v7h2V4c0-1.1-.9-2-2-2zm0 18h-7v2h7c1.1 0 2-.9 2-2v-7h-2v7zM4 13H2v7c0 1.1.9 2 2 2h7v-2H4v-7z"></path></g>
 </if>
       <g id="warning"><path d="M1 21h22L12 2 1 21zm12-3h-2v-2h2v2zm0-4h-2v-4h2v4z"></path></g>
 <if expr="not chromeos">
       <g id="web"><path d="M20 4H4c-1.1 0-1.99.9-1.99 2L2 18c0 1.1.9 2 2 2h16c1.1 0 2-.9 2-2V6c0-1.1-.9-2-2-2zm-5 14H4v-4h11v4zm0-5H4V9h11v4zm5 5h-4V9h4v9z"></path></g>
 </if>
     </defs>
   </svg>
 </iron-iconset-svg>