Purpose: This is a unit test method refactoring to try to
improve readability; I made this when adding unit tests for
https://chromium-review.googlesource.com/c/419148/.

In this CL:
 - Extract ExampleChange helper method
 - Explicitly write names of args for DoPresubmitChecks
 - Other minor changes to make the style more consistent

Change-Id: I52236e285e50db890245c6c4b69c70ddf258f140
Reviewed-on: https://chromium-review.googlesource.com/419184Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Quinten Yearsley <qyearsley@chromium.org>