[svg] Document container size semantics

Also add a helper for accessing the root SVG element:
SkSVGDOM::getRoot().

Bug: skia:11144
Change-Id: Icc7af8f0521203de26836e56dee8c503a56997c0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/415746
Commit-Queue: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@google.com>
Reviewed-by: Tyler Denniston <tdenniston@google.com>
1 file changed