Add non-member non-mutating methods for common gfx::Rect operations.

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 037b9d6b5b64ea1509fabe62ae322735bfe073f6..c064cc810f6443c5d8061381694e7311d2c909f0 100644
--- a/ui/views/animation/bounds_animator.cc
+++ b/ui/views/animation/bounds_animator.cc
@@ -235,8 +235,7 @@ 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 = view->bounds();
-    total_bounds.Union(new_bounds);
+    gfx::Rect total_bounds = gfx::UnionRects(new_bounds, view->bounds());
 
     // Build up the region to repaint in repaint_bounds_. We'll do the repaint
     // when all animations complete (in AnimationContainerProgressed).