Fix diff generation by difflib to report missing end of file newlines.

The diff generated by the difflib is incorrect if one of the files
does not have a newline at the end of the file and the last lines
are different. Relevant Python issue: http://bugs.python.org/issue2142

BUG=398435
TEST=test-webkitpy

Review URL: https://codereview.chromium.org/423073004

git-svn-id: svn://svn.chromium.org/blink/trunk@180764 bbb929c8-8fbe-4397-9dbb-9b2b20218538