MD Settings: Add description for Protected Content

chrome/app/settings_strings.grdp

 <?xml version="1.0" encoding="utf-8"?>
 <!-- Settings-specific strings (included from generated_resources.grd). -->
 <grit-part>
   <!-- About Page -->
   <message name="IDS_SETTINGS_ABOUT_PAGE_BROWSER_VERSION" desc="The text label describing the version of the browser, example: Version 57.0.2937.0 (Developer Build) unknown (64-bit)">
     Version <ph name="PRODUCT_VERSION">$1<ex>15.0.865.0</ex></ph> (<ph name="PRODUCT_CHANNEL">$2<ex>Developer Build</ex></ph>) <ph name="PRODUCT_MODIFIER">$3</ph> <ph name="PRODUCT_VERSION_BITS">$4</ph>
   </message>
   <if expr="chromeos">
     <message name="IDS_SETTINGS_ABOUT_PAGE_DETAILED_BUILD_INFO" desc="Label describing the section that shows detailed information about the current Chrome build.">
       Detailed build information
@@ skipping 1738 matching lines @@
   </message>
   <message name="IDS_SETTINGS_SITE_SETTINGS_PDF_DIFFERENT_APPLICATION" desc="Label for opening PDF documents in a different application.">
     Open PDFs using a different application
   </message>
   <message name="IDS_SETTINGS_SITE_SETTINGS_POPUPS" desc="Label for the popups site settings.">
     Popups
   </message>
   <message name="IDS_SETTINGS_SITE_SETTINGS_PROTECTED_CONTENT" desc="Label for the protected content site settings.">
     Protected Content
   </message>
+  <message name="IDS_SETTINGS_SITE_SETTINGS_PROTECTED_CONTENT_EXPLANATION" desc="Text that is displayed on the Protected Content section of Content Settings. This text explains that enabling protected content may require using a uniquely identifiable machine identifier.">
+    Some content services use unique identifiers for the purposes of authorizing access to protected content.
+  </message>
   <message name="IDS_SETTINGS_SITE_SETTINGS_PROTECTED_CONTENT_ENABLE" desc="Label for the toggle enabling protected content.">
     Allow identifiers for protected content (computer restart may be required)
   </message>
   <message name="IDS_SETTINGS_SITE_SETTINGS_UNSANDBOXED_PLUGINS" desc="Label for the unsandboxed plugin access site settings.">
     Unsandboxed plugin access
   </message>
   <message name="IDS_SETTINGS_SITE_SETTINGS_USB_DEVICES" desc="Label for the USB devices in site settings.">
     USB devices
   </message>
   <message name="IDS_SETTINGS_SITE_SETTINGS_ZOOM_LEVELS" desc="Label for the Zoom levels category in site settings.">
@@ skipping 1031 matching lines @@
       System
     </message>
     <message name="IDS_SETTINGS_SYSTEM_HARDWARE_ACCELERATION_LABEL" desc="Label for the checkbox that forces Chrome to render via hardware acceleration (GPU) when available.">
       Use hardware acceleration when available
     </message>
     <message name="IDS_SETTINGS_SYSTEM_PROXY_SETTINGS_BUTTON" desc="Text for the button that changes a user's network proxy settings.">
       Change proxy settings
     </message>
   </if>
 </grit-part>