Fix bug of the destructor of SkSVGDevice

When SkSVGDevice owns `fWriter`, we must ensure that `fRootElement`
is destructed before `fWriter`. Otherwise, while `fRootElement` is
destrcuting, it tries to write data into `fWriter` who is alreay
deleted.

This bug is introduced by
https://skia-review.googlesource.com/c/skia/+/192040.

Change-Id: Idbb998a3c25f6e0d731e5070fd9f9bdde3afcd1a
Reviewed-on: https://skia-review.googlesource.com/c/192301
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>

src/svg/SkSVGDevice.cpp

@@ -664,6 +664,7 @@ SkSVGDevice::SkSVGDevice(const SkISize& size, SkXMLWriter* writer, bool ownsWrit
 
 SkSVGDevice::~SkSVGDevice() {
     if (fOwnsWriter && fWriter) {
+        fRootElement.reset(nullptr);
         delete fWriter;
     }
 }