Remove the ability of webpages to specify strings for the onbeforeunload dialog.

components/app_modal_strings.grdp

 <?xml version="1.0" encoding="utf-8"?>
 <grit-part>
 
   <!-- JavaScript Dialog Box strings -->
   <message name="IDS_JAVASCRIPT_MESSAGEBOX_TITLE" desc="Title for JavaScript prompt and confirm originating from a webpage">
     <ph name="SITE">$1<ex>http://www.google.com</ex></ph> says:
   </message>
   <message name="IDS_JAVASCRIPT_MESSAGEBOX_TITLE_IFRAME" desc="Title for JavaScript prompt and confirm originating from an iframe inside the webpage">
     An embedded page at <ph name="SITE">$1<ex>http://www.google.com</ex></ph> says:
   </message>
   <message name="IDS_JAVASCRIPT_MESSAGEBOX_TITLE_NONSTANDARD_URL" desc="Title for JavaScript prompt and confirm originating from a webpage with a non-standard URL such as |data:|">
     This page says:
   </message>
   <message name="IDS_JAVASCRIPT_MESSAGEBOX_TITLE_NONSTANDARD_URL_IFRAME" desc="Title for JavaScript prompt and confirm originating from an iframe inside the webpage with a non-standard URL such as |data:|">
     An embedded page on this webpage says:
   </message>
   <message name="IDS_JAVASCRIPT_MESSAGEBOX_SUPPRESS_OPTION" desc="Optional UI shown on the message box, in the form of a checkbox, allowing the user to suppress additional message boxes from the page.">
     Prevent this page from creating additional dialogs.
   </message>
 
   <!-- "Before Unload" Dialog Box strings -->
   <message name="IDS_BEFOREUNLOAD_MESSAGEBOX_TITLE" desc="Title for the 'before unload' dialog.">
     Confirm Navigation
   </message>
-  <message name="IDS_BEFOREUNLOAD_MESSAGEBOX_FOOTER" desc="Text shown at the bottom of the dialog, after the message provided by the script.">
-    Are you sure you want to leave this page?
+  <message name="IDS_BEFOREUNLOAD_MESSAGEBOX_MESSAGE" desc="Text shown as the message of the 'before unload' dialog.">
+    This page is warning you that if you leave this page, data may be lost. Are you sure you want to leave this page?
   </message>
   <message name="IDS_BEFOREUNLOAD_MESSAGEBOX_OK_BUTTON_LABEL" desc="The text on the button which navigates the user away from the page.">
     Leave this Page
   </message>
   <message name="IDS_BEFOREUNLOAD_MESSAGEBOX_CANCEL_BUTTON_LABEL" desc="The text on the button which cancels the navigation away from the page.">
     Stay on this Page
   </message>
 
   <!-- "Before Reload" Dialog Box strings (same as "Before Unload" but when reloading rather than unloading the page -->
   <message name="IDS_BEFORERELOAD_MESSAGEBOX_TITLE" desc="Title for the 'before reload' dialog.">
     Confirm Reload
   </message>
-  <message name="IDS_BEFORERELOAD_MESSAGEBOX_FOOTER" desc="Text shown at the bottom of the dialog, after the message provided by the script.">
-    Are you sure you want to reload this page?
+  <message name="IDS_BEFORERELOAD_MESSAGEBOX_MESSAGE" desc="Text shown as the message of the 'before reload' dialog.">

[Charlie Reis, 2016/02/18 21:37:13]

nit: 'before unload' dialog for reloads.

(There's no beforereload event, which is what this sounds like.)

[Avi (use Gerrit), 2016/02/18 21:42:26]

Done.

+    This page is warning you that if you reload this page, data may be lost. Are you sure you want to reload this page?
   </message>
   <message name="IDS_BEFORERELOAD_MESSAGEBOX_OK_BUTTON_LABEL" desc="The text on the button which reloads the page.">
     Reload this Page
   </message>
   <message name="IDS_BEFORERELOAD_MESSAGEBOX_CANCEL_BUTTON_LABEL" desc="The text on the button which cancels the page reload.">
     Don't Reload
   </message>
 
 </grit-part>