[SVG] Replace custom attribute value wrappers with std::variant

Less boilerplate, more STL.

Change-Id: I601f75877d60085cbf3d39f401543fbe9c086f90
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/282836
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
37 files changed