WebUI: update empty-state strings for MD pages.

chrome/app/bookmarks_strings.grdp

 <?xml version="1.0" encoding="utf-8"?>
 <!-- Bookmarks specific strings (included from generated_resources.grd). -->
 <grit-part>
   <!-- Begin of Bookmarks Bar strings-->
   <if expr="not is_android">
     <!-- The following two strings are shown when the user has no bookmarks yet. -->
     <message name="IDS_BOOKMARKS_NO_ITEMS" desc="Text shown when the user has no bookmarks">
       For quick access, place your bookmarks here on the bookmarks bar.
     </message>
     <message name="IDS_BOOKMARK_BAR_IMPORT_LINK" desc="text shown for importing ">
@@ skipping 366 matching lines @@
   <message name="IDS_MD_BOOKMARK_MANAGER_ADD_BOOKMARK_TITLE" desc="Title of the dialog in the bookmark manager that creates a new bookmark.">
     Add bookmark
   </message>
   <message name="IDS_MD_BOOKMARK_MANAGER_ADD_FOLDER_TITLE" desc="Title of the dialog in the bookmark manager that creates a new bookmark folder.">
     Add folder
   </message>
   <message name="IDS_MD_BOOKMARK_MANAGER_CLEAR_SEARCH" desc="Title of the button in the bookmark manager that stops a search.">
     Clear search
   </message>
   <message name="IDS_MD_BOOKMARK_MANAGER_EMPTY_LIST" desc="Text on the bookmarks list that indicates that a folder has no bookmarks.">
-    This folder is empty
+    To bookmark pages, click the star in the address bar

[dschuyler, 2017/06/01 20:37:50]

Consider mentioning the desc that "star" is describing the bookmark icon in the
omnibox.

[scottchen, 2017/06/01 20:44:58]

Done.

   </message>
   <message name="IDS_MD_BOOKMARK_MANAGER_FOLDER_RENAME_TITLE" desc="Title of the bookmark editor window when editing folders.">
     Rename Folder
   </message>
   <message name="IDS_MD_BOOKMARK_MANAGER_MENU_ADD_BOOKMARK" desc="Title of the bookmark toolbar dropdown menu item that adds a new bookmark.">
     Add new bookmark
   </message>
   <message name="IDS_MD_BOOKMARK_MANAGER_MENU_ADD_FOLDER" desc="Title of the bookmark toolbar dropdown menu item that adds a new folder.">
     Add new folder
   </message>
@@ skipping 49 matching lines @@
   </if>
   <!-- End of Bookmarks Menu (in the Main Menu) strings. -->
 
   <!-- Begin of Bookmark Star button strings. -->
   <message name="IDS_TOOLTIP_STARRED" desc="The tooltip shown when you hover the star button in the right side of the location bar and the current page is bookmarked.">
     Edit bookmark for this page
   </message>
   <!-- End of Bookmark Star button strings. -->
 
 </grit-part>