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..d64eaa6c8deedfe3c14e54e1f682e6d32e8140b7 100644
--- a/Tools/Scripts/webkitpy/common/system/filesystem_mock.py
+++ b/Tools/Scripts/webkitpy/common/system/filesystem_mock.py
@@ -391,7 +391,9 @@ class MockFileSystem(object):
         source = self.normpath(source)
         destination = self.normpath(destination)
 
-        for source_file in self.files:
+        #Have to make a copy because we are modifying the list within the loop

[vivekg, 2014/08/22 08:48:09]

nit: space after '#'

[patro, 2014/08/22 09:28:28]

Done.

+        file_list = self.files.keys()
+        for source_file in file_list:

[vivekg, 2014/08/22 08:48:09]

or could be `for source_file in self.files.keys():`

[patro, 2014/08/22 09:28:28]

Done.

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