Stocks FAB "+" sometimes teleports to near the top left

The matrix argument to drawPicture doesn't seem to do what we want exactly.
Instead, use the normal matrix in the canvas. Also, handle cull rects with
non-zero left and top coordinates.

Fixes #1229

flow/raster_cache.cc

@@ -84,8 +84,9 @@ skia::RefPtr<SkImage> RasterCache::GetPrerolledImage(GrContext* context,
       if (surface) {
         SkCanvas* canvas = surface->getCanvas();
         canvas->clear(SK_ColorTRANSPARENT);
-        SkMatrix matrix = SkMatrix::MakeScale(scaleX, scaleY);
-        canvas->drawPicture(picture, &matrix, nullptr);
+        canvas->translate(-rect.left(), -rect.right());
+        canvas->scale(scaleX, scaleY);
+        canvas->drawPicture(picture);
         entry.image = skia::AdoptRef(surface->newImageSnapshot());
       }
     }