Fix runtime error in copytree() MockFileSystem

Tools/Scripts/webkitpy/common/system/filesystem_mock.py

Index: Tools/Scripts/webkitpy/common/system/filesystem_mock.py
diff --git a/Tools/Scripts/webkitpy/common/system/filesystem_mock.py b/Tools/Scripts/webkitpy/common/system/filesystem_mock.py
index 70dfff3fa64287e63b077decfda3cf5b49f3d7ee..919bde53b8afbf6f7b2927efcc99e5f6b40a7c8d 100644
--- a/Tools/Scripts/webkitpy/common/system/filesystem_mock.py
+++ b/Tools/Scripts/webkitpy/common/system/filesystem_mock.py
@@ -391,7 +391,7 @@ class MockFileSystem(object):
         source = self.normpath(source)
         destination = self.normpath(destination)
 
-        for source_file in self.files:
+        for source_file in self.files.keys():

[Dirk Pranke, 2014/08/27 23:42:45]

this change is fine for Python 2.7, but I think it will have the same problem in
python 3 that the 'for ... in dict' old version does, so it might be better to 
change this to "for source_file in list(self.files)", which is safe in Python 3.

Not that we actually support Python 3 yet, of course ...

[patro, 2014/09/01 09:14:02]

Done.

             if source_file.startswith(source):
                 destination_path = self.join(destination, self.relpath(source_file, source))
                 self.maybe_make_directory(self.dirname(destination_path))