Added strings for notification when user presses backspace.

ui/strings/ui_strings.grd

 <?xml version="1.0" encoding="UTF-8"?>
 
 <!-- This file contains definitions of resources that will be translated for
 each locale. Specifically, these are UI strings that are used by app/ that
 need to be translated for each locale.-->
 
 <grit base_dir="." latest_public_release="0" current_release="1"
       source_lang_id="en" enc_check="möl">
   <outputs>
     <!-- TODO add each of your output files.  Modify the three below, and add
@@ skipping 491 matching lines @@
       </message>
       <message name="IDS_APP_DOWN_ARROW_KEY" desc="Down arrow key">
         Down Arrow
       </message>
       <message name="IDS_APP_ENTER_KEY" desc="Enter key">
         Enter
       </message>
       <message name="IDS_APP_SPACE_KEY" desc="Space bar key">
         Space
       </message>
+      <message name="IDS_APP_ALT_KEY" desc="Alt key">
+        Alt
+      </message>
       <message name="IDS_APP_F1_KEY" desc="F1 key">
         F1
       </message>
       <message name="IDS_APP_F11_KEY" desc="F11 key">
         F11
       </message>
       <message name="IDS_APP_BACKSPACE_KEY" desc="Backspace key">
         Backspace
       </message>
       <message name="IDS_APP_COMMA_KEY" desc="Comma key">
@@ skipping 167 matching lines @@
       </message>
       <message name="IDS_APP_LIST_SPEECH_HINT_TEXT" desc="The text label in the speech recognition UI to ask the user to speak the search query">
         Speak now
       </message>
       <message name="IDS_APP_LIST_SPEECH_NETWORK_ERROR_HINT_TEXT" desc="The text label in the speech recognition UI to show the speech recognition can't start because of network error.">
         No internet connection
       </message>
     </messages>
   </release>
 </grit>