Fix runtime error in copytree() MockFileSystem

We are iterating over a list and modifying the same list within the loop which causes a RunTimeError.
Iterate over keys instead of the list itself to prevent this.

Review URL: https://codereview.chromium.org/493413002

git-svn-id: svn://svn.chromium.org/blink/trunk@181177 bbb929c8-8fbe-4397-9dbb-9b2b20218538