Fix bug where custom cursors would dissappear when screen was rotated

ui/gfx/skbitmap_operations.cc

Index: ui/gfx/skbitmap_operations.cc
diff --git a/ui/gfx/skbitmap_operations.cc b/ui/gfx/skbitmap_operations.cc
index a12ef83f5b2fec4be1467e9c8eef71fbb44fd996..956b4f3182b3160faaab6a3df06dc552c199a5d9 100644
--- a/ui/gfx/skbitmap_operations.cc
+++ b/ui/gfx/skbitmap_operations.cc
@@ -757,6 +757,9 @@ SkBitmap SkBitmapOperations::CreateDropShadow(
 // static
 SkBitmap SkBitmapOperations::Rotate(const SkBitmap& source,
                                     RotationAmount rotation) {
+  // SkCanvas::drawBitmap() fails silently with unpremultiplied SkBitmap.
+  DCHECK(source.info().alphaType() != kUnpremul_SkAlphaType);
+
   SkBitmap result;
   SkScalar angle = SkFloatToScalar(0.0f);