Add helper for computing the reflected control point for smooth segments
Add helper for computing the reflected control point for smooth segments

BUG=467592

Review URL: https://codereview.chromium.org/1019853003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@192177 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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