[svg] Allow clients to pass a custom SkFontMgr

Introduce a Builder helper and plumb the client-provided SkFontMgr for
font resolution.

Also clean up some of the legacy SkSVGDom factories.

Bug: skia:10840
Change-Id: I6e1eabe7c257cb75dfdb5bf67054f93f25769027
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/333577
Commit-Queue: Florin Malita <fmalita@google.com>
Reviewed-by: Tyler Denniston <tdenniston@google.com>
10 files changed