Make gfx::Rect class operations consistently mutate the class they are called on.

ppapi/utility/graphics/paint_aggregator.cc

Index: ppapi/utility/graphics/paint_aggregator.cc
diff --git a/ppapi/utility/graphics/paint_aggregator.cc b/ppapi/utility/graphics/paint_aggregator.cc
index d4fc72165b9323450434dfe9b529c5c579f20106..29b726c310c8f0948ce4afc205c73f5206b671ad 100644
--- a/ppapi/utility/graphics/paint_aggregator.cc
+++ b/ppapi/utility/graphics/paint_aggregator.cc
@@ -207,7 +207,7 @@ Rect PaintAggregator::ScrollPaintRect(const Rect& paint_rect,
   Rect result = paint_rect;
 
   result.Offset(amount);
-  result = update_.scroll_rect.Intersect(result);
+  result.Intersect(update_.scroll_rect);
 
   // Subtract out the scroll damage rect to avoid redundant painting.
   return result.Subtract(update_.GetScrollDamage());