Make sure files end with exactly one new line and police this in presubmit.

tools/presubmit.py

 #!/usr/bin/env python
 #
 # Copyright 2012 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
@@ skipping 344 matching lines @@
         parts.pop()
       for part in parts:
         line += part.count('\n') + 1
         lines.append(str(line))
       linenumbers = ', '.join(lines)
       if len(lines) > 1:
         print "%s has trailing whitespaces in lines %s." % (name, linenumbers)
       else:
         print "%s has trailing whitespaces in line %s." % (name, linenumbers)
       result = False
+    if not contents.endswith('\n') or contents.endswith('\n\n'):
+      print "%s does not end with a single new line." % name
+      result = False
     # Check two empty lines between declarations.
     if name.endswith(".cc"):
       line = 0
       lines = []
       parts = contents.split('\n')
       while line < len(parts) - 2:
         if self.EndOfDeclaration(parts[line]):
           if self.StartOfDeclaration(parts[line + 1]):
             lines.append(str(line + 1))
             line += 1
@@ skipping 48 matching lines @@
         "two empty lines between declarations check..."
   success = SourceProcessor().Run(workspace) and success
   if success:
     return 0
   else:
     return 1
 
 
 if __name__ == '__main__':
   sys.exit(Main())