Extract diff_text function out of the Port class.

third_party/WebKit/Tools/Scripts/webkitpy/common/unified_diff.py

+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+"""A utility function to do text diffs of expected and actual layout test results."""
+
+import difflib
+
+
+def unified_diff(expected_text, actual_text, expected_filename, actual_filename):
+    """Returns a string containing the diff of the two text strings
+    in 'unified diff' format.
+    """
+    # The filenames show up in the diff output, make sure they're
+    # raw bytes and not unicode, so that they don't trigger join()
+    # trying to decode the input.
+    expected_filename = _to_raw_bytes(expected_filename)
+    actual_filename = _to_raw_bytes(actual_filename)
+
+    diff = difflib.unified_diff(
+        expected_text.splitlines(True),
+        actual_text.splitlines(True),
+        expected_filename,
+        actual_filename)
+
+    return ''.join(_diff_fixup(diff))
+
+
+def _to_raw_bytes(string_value):
+    if isinstance(string_value, unicode):
+        return string_value.encode('utf-8')
+    return string_value
+
+
+def _diff_fixup(diff):
+    # 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 it is present in
+    # the diff. Relevant Python issue: http://bugs.python.org/issue2142
+    for line in diff:
+        yield line
+        if not line.endswith('\n'):
+            yield '\n\\ No newline at end of file\n'