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This change migrates the "order" SVG attribute animations over to
SVGIntegerOptionalIntegerInterpolationType and enables additive keyframe
composite modes as well as neutral keyframes.

This is a rebased reupload of: https://codereview.chromium.org/1417483015/

BUG=530436

Review URL: https://codereview.chromium.org/1416703004

Cr-Commit-Position: refs/heads/master@{#361289}