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1) A bug introduced during addressing review feedbacks so now the split
files can't be split further because they are wrong.

2) The script doesn't work if one if the four switches are missing. This
is problematic because we could easily split into a state where certain
switches no longer have values.

Added unit tests to make sure the above two issues are covered.

BUG=932197
TEST=unittests
R=askitkine@chromium.org

Change-Id: Ic961f609f0a0d00ebf213206a70b4fb76b83aac8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1497797Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Zhenyao Mo <zmo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638262}