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

ash/wm/workspace/workspace_window_resizer.cc

Index: ash/wm/workspace/workspace_window_resizer.cc
diff --git a/ash/wm/workspace/workspace_window_resizer.cc b/ash/wm/workspace/workspace_window_resizer.cc
index d3cc9b7c116d2ec577c3c7e3b74edab9c961bbff..82c7f6c12b7efbfca1cfa7b283b137353a61ea50 100644
--- a/ash/wm/workspace/workspace_window_resizer.cc
+++ b/ash/wm/workspace/workspace_window_resizer.cc
@@ -688,8 +688,8 @@ void WorkspaceWindowResizer::UpdateDragPhantomWindow(const gfx::Rect& bounds,
   const gfx::Rect root_bounds_in_screen(another_root->GetBoundsInScreen());
   const gfx::Rect bounds_in_screen =
       ScreenAsh::ConvertRectToScreen(window()->parent(), bounds);
-  gfx::Rect bounds_in_another_root = root_bounds_in_screen;
-  bounds_in_another_root.Intersect(bounds_in_screen);
+  gfx::Rect bounds_in_another_root =
+      gfx::Intersection(root_bounds_in_screen, bounds_in_screen);
 
   const float fraction_in_another_window =
       (bounds_in_another_root.width() * bounds_in_another_root.height()) /