Resolve computed 'transform-origin' for SVG elements

Make the local helper ReferenceBoxForTransform available via
ComputedStyleUtils and use it in TransformOrigin::CSSValueFromComp...
Replace usages of MinimumValueForLength with FloatValueForLength. This
matches what is done when resolving/applying transform in the general
case.
Also sink the common handling of the 'z' component out of the 'if'.

Bug: 740300
Change-Id: Id0e6cbcc85942ccc8cdb3f8ed86ecc6e7aaf492e
Reviewed-on: https://chromium-review.googlesource.com/c/1382434
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617548}
4 files changed