[SVGCanvas] Clean up public factories

Remove the internal-only/XMLWriter-based factory.

Update SkSVGDevice to always own the xml writer.

The only internal client passing an interesting XMLWriter is
SVGDeviceTest - update to use the device factory directly.

While at it, update the SkSVGDevice factory to return smart pointers
(Create -> Make).

Change-Id: Ibda1ca86ef9fb81ab512822000835ace1af67978
Reviewed-on: https://skia-review.googlesource.com/c/192580
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
7 files changed