Decouple SVG from SkFontMgr::RefDefault()

Instead of relying on the default SkFontMgr as a backup, this
explicitly sets the SkFontMgr to the test one (in our tests)
and uses and empty SkFontMgr if one was not set (e.g. by clients).

Change-Id: I1ec4c710ec425be93ebe0d792ded172ea76838d2
Bug: b/305780908
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/772660
Reviewed-by: Florin Malita <fmalita@google.com>
13 files changed