Fix DM race in SVGSrc

Fix DM race in SVGSrc

The SVG source currently attempts to build the DOM lazily, in response
to draw() or size() calls.  But apprently DM may call these
concurrently.

Build the DOM in the constructor instead, to avoid races (and the need
for synchronization).

R=robertphillips@google.com,mtklein@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2482123003

Review-Url: https://codereview.chromium.org/2482123003