add legacy arcto flag
add legacy arcto flag

Skia used quadratics to approximate the arc specified
by the pt/pt/radius version of arcTo(), which in turn
is called by <canvas>.

A future Skia CL will replace this with a conic
implmentation to match circles and oval-based arcs.

This adds a legacy flag to keep the old behavior until
the changed layout test results can be rebaselined.

NOTRY=true
R=fmalita@chromium.org
BUG=578885

Review URL: https://codereview.chromium.org/1610773002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#370471}

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