Remove AT::blend, TM::isBackFaceVisible, and FloatPolygon::overlapsRect

This patch removes the following dead math code:
1) AffineTransform::blend
2) TransformationMatrix::isBackFaceVisible - removed in wkrev.com/145442
3) FloatPolygon::overlapsRect

No tests as there is no change in behavior.

Review URL: https://codereview.chromium.org/438123002

git-svn-id: svn://svn.chromium.org/blink/trunk@179758 bbb929c8-8fbe-4397-9dbb-9b2b20218538