Web Animations: Add SVGNumberInterpolationType

This change migrates the following SVG attribute animations over to
SVGNumberInterpolationType and enables additive keyframe composite modes
as well as neutral keyframes:
amplitude, azimuth, bias, diffuseConstant, divisor, elevation, exponent,
intercept, k1, k2, k3, k4, limitingConeAngle, offset, pathLength,
pointsAtX, pointsAtY, pointsAtZ, scale, seed, slope, specularConstant,
specularExponent, surfaceScale, z.

BUG=530436

Review URL: https://codereview.chromium.org/1410643011

Cr-Commit-Position: refs/heads/master@{#357454}
29 files changed