media_controls: Add UI for showing a promo bubble on a control button.

content/app/strings/content_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 content that
 need to be translated for each locale.-->
 
 <!-- Some of these strings and string descriptions were taken from
 WebKit/win/WebCoreLocalizedStrings.cpp so we include the original license
 below:
 
@@ skipping 153 matching lines @@
       change nontranslateable parts of the messages into placeholders (using the
       <ph> element).  You can also use the 'grit add' tool to help you identify
       nontranslateable parts and create placeholders for them. -->
 
       <message name="IDS_DETAILS_WITHOUT_SUMMARY_LABEL" desc="Clickable label to show detailed information or to hide the detailed information.">
         Details
       </message>
       <message name="IDS_DOWNLOAD_BUTTON_LABEL" desc="Clickable button label to download media file.">
         Save
       </message>
+      <message name="IDS_DOWNLOAD_BUTTON_PROMO_LABEL" desc="Text for the download button promo UI.">
+        Download videos to watch later
+      </message>
       <message name="IDS_SEARCHABLE_INDEX_INTRO" desc="Text that appears at the start of nearly-obsolete webpages in the form of a 'searchable index'.">
         This is a searchable index. Enter search keywords: '''
       </message>
       <message name="IDS_FORM_CALENDAR_CLEAR" desc="Label for a button which clears a date input field.">
         Clear
       </message>
       <message name="IDS_FORM_CALENDAR_TODAY" desc="Label for a button which sets today to a date input field.">
         Today
       </message>
       <message name="IDS_FORM_DATE_FORMAT_DAY_IN_MONTH" desc="Short word to indicate which is the day-in-month part in a date format. e.g. 'Day' in 'Month/Day/Year'">
@@ skipping 702 matching lines @@
         Off
       </message>
 
       <message name="IDS_PLUGIN_INITIALIZATION_ERROR" desc="A message displayed when a plugin failed to load">
         Couldn't load plugin.
       </message>
 
     </messages>
   </release>
 </grit>