Add some comments to Settings.in
Add some comments to Settings.in

See https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/984HMZyFQNo
for discussion.

Review URL: https://codereview.chromium.org/26480002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@159115 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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