[svg] Rename gradient units type to object bounding box units

It turns out that "userSpaceOnUse | objectBoundingBox" can be specified
as the unit type for multiple elements, such as clips, masks, filters
etc. and is not specific to gradients.

The full list:
https://www.w3.org/TR/SVG11/coords.html#ObjectBoundingBoxUnits

Bug: skia:10842
Change-Id: I995d588862fb43b9f130f0455d7af8de47046af4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/330616
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Tyler Denniston <tdenniston@google.com>
9 files changed