[svg] Document container size semantics
[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 <a8262bf13ea8c26821e91ea9bca6740a77c96dcd@chromium.org>
Commit-Queue: Florin Malita <a8262bf13ea8c26821e91ea9bca6740a77c96dcd@google.com>
Reviewed-by: Tyler Denniston <293ae2d31b7e9da32c4fb58bdaaed1f4bef4c6e7@google.com>

aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia