Use SaveLayerAlpha bounds when doing Save-Draw-Restore optimization

cc/paint/display_item_list.cc

Index: cc/paint/display_item_list.cc
diff --git a/cc/paint/display_item_list.cc b/cc/paint/display_item_list.cc
index b947403a73883fdc2ee0c6d6b05d2d1b6771fccc..cd3583bda8c17540813e47bdbfbbd535731e0797 100644
--- a/cc/paint/display_item_list.cc
+++ b/cc/paint/display_item_list.cc
@@ -183,7 +183,8 @@ static bool MergeAndDrawIfPossible(const CompositingDisplayItem& save_item,
   if (!op->IsDrawOp())
     return false;
 
-  op->RasterWithAlpha(canvas, save_item.alpha);
+  SkRect bounds = save_item.has_bounds ? save_item.bounds : PaintOp::kUnsetRect;
+  op->RasterWithAlpha(canvas, bounds, save_item.alpha);
   return true;
 }