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 519 matching lines @@
       <message name="IDS_APP_MEDIA_PLAY_PAUSE_KEY" desc="Media play pause key">
         Media Play/Pause
       </message>
       <message name="IDS_APP_MEDIA_PREV_TRACK_KEY" desc="Media previous track key">
         Media Previous Track
       </message>
       <message name="IDS_APP_MEDIA_STOP_KEY" desc="Media stop key">
         Media Stop
       </message>
 
+      <!-- Shortcut Keys -->

[Peter Kasting, 2016/05/19 09:19:00]

Nit: Why does this get its own heading?  Shouldn't it just be another one in the
"Key names" list above?

[Matt Giuca, 2016/05/19 10:27:01]

Oh yes, you're right. I don't have access to my git repo at the moment. Can you
leave me an LG and I'll fix this nit before submitting.

[Matt Giuca, 2016/05/19 23:11:53]

Done.

+      <message name="IDS_APP_ALT_KEY" desc="Alt key">
+        Alt
+      </message>
+
       <!-- Shortcut Modifiers -->
       <message name="IDS_APP_CONTROL_MODIFIER" desc="Control key shortcut modifier">
         Ctrl+<ph name="KEY_COMBO_NAME">$1<ex>C</ex></ph>
       </message>
       <message name="IDS_APP_ALT_MODIFIER" desc="Alt key shortcut modifier">
         Alt+<ph name="KEY_COMBO_NAME">$1<ex>C</ex></ph>
       </message>
       <message name="IDS_APP_SHIFT_MODIFIER" desc="Shift key shortcut modifier">
         Shift+<ph name="KEY_COMBO_NAME">$1<ex>C</ex></ph>
       </message>
@@ skipping 139 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>