Replace use of SkLerpXfermode with SkArithmeticMode
Replace use of SkLerpXfermode with SkArithmeticMode

SkLerpXermode is going away. The same functionality can be
achieved with SkArithmeticMode.

Review URL: https://codereview.chromium.org/1610573004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#370692}

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