[svg] Implement gradientUnits="objectBoundingBox"

Currently only works properly when filling rects, since that is the only
node that implements bounds computation with this CL.

Summary of changes:

- Scale gradient coords by object bounds when units are
  kObjectBoundingBox.
- Make fGradientUnits protected instead of private.
- Change default value of fGradientUnits to kObjectBoundingBox, which
  is the default according to the spec.
- Change SkSVGNode::onObjectBoundingBox to take a full render context
  instead of length context.
- Implement bounds computation for SkSVGRect, SkSVGContainer and
  SkSVGUse.

Bug: skia:10842
Change-Id: I2e999985e67644e50da7f681fde190bcf4823eec
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/329223
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Tyler Denniston <tdenniston@google.com>
14 files changed