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An upcoming CL in Skia uses SIMD instructions to
compute the coefficients for cubic and quadratic
polynomials. This introduces some minor differences.

Add a legacy flag to permit cubic eval() to work
as it did in the past until the appropriate
layout tests can be rebaselined.

R=fmalita@chromium.org

Review URL: https://codereview.chromium.org/1632313002

Cr-Commit-Position: refs/heads/master@{#371606}