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

ui/views/animation/bounds_animator.cc

Index: ui/views/animation/bounds_animator.cc
diff --git a/ui/views/animation/bounds_animator.cc b/ui/views/animation/bounds_animator.cc
index 2c3fd053543b8d2f1461b0fe0f8471691e2bdfdb..037b9d6b5b64ea1509fabe62ae322735bfe073f6 100644
--- a/ui/views/animation/bounds_animator.cc
+++ b/ui/views/animation/bounds_animator.cc
@@ -235,14 +235,12 @@ void BoundsAnimator::AnimationProgressed(const Animation* animation) {
   gfx::Rect new_bounds =
       animation->CurrentValueBetween(data.start_bounds, data.target_bounds);
   if (new_bounds != view->bounds()) {
-    gfx::Rect total_bounds = new_bounds.Union(view->bounds());
+    gfx::Rect total_bounds = view->bounds();
+    total_bounds.Union(new_bounds);
 
     // Build up the region to repaint in repaint_bounds_. We'll do the repaint
     // when all animations complete (in AnimationContainerProgressed).
-    if (repaint_bounds_.IsEmpty())
-      repaint_bounds_ = total_bounds;
-    else
-      repaint_bounds_ = repaint_bounds_.Union(total_bounds);
+    repaint_bounds_.Union(total_bounds);
 
     view->SetBoundsRect(new_bounds);
   }