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

This reverts commit 97e0861b677c833b2cb9886c125a79f442c271df.

Reason for revert: not all builders support C++17 STL

Original change's description:
> [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>

TBR=fmalita@chromium.org,reed@google.com,tdenniston@google.com

Change-Id: If5816f11e9cf85dcec2452229c5675037303b220
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/282847
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
37 files changed