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This defines WebScrollOffsetAnimationCurveImpl, which wraps a
cc::ScrollOffsetAnimationCurve. This is needed for
implementing the CSSOM Smooth Scroll API.

BUG=243871

Review URL: https://codereview.chromium.org/112933005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245645 0039d316-1c4b-4281-b951-d872f2087c98