Correctly initialize LayerIter's cached device origin.

Previously, LayerIter called the base device iterator's next() call
during initialization. However, its next() call should be called instead,
since it does both the base device iterator.next() AND caches the
device's reported origin.

Without calling this->next(), the first device's origin would never be
fetched and the layer would report an incorrect origin.

Bug: b/151328374
Change-Id: I889673e2e93e54618544d62333bd9d04438bea00
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/279420
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Derek Sollenberger <djsollen@google.com>

src/core/SkCanvas.cpp

@@ -2969,7 +2969,9 @@ SkCanvas::LayerIter::LayerIter(SkCanvas* canvas) {
     SkASSERT(canvas);
 
     fImpl = new (fStorage) SkDrawIter(canvas);
-    fDone = !fImpl->next();
+    // This advances the base iterator to the first device and caches its origin,
+    // correctly handling the case where there are no devices.
+    this->next();
 }
 
 SkCanvas::LayerIter::~LayerIter() {