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

ash/wm/partial_screenshot_view.cc

Index: ash/wm/partial_screenshot_view.cc
diff --git a/ash/wm/partial_screenshot_view.cc b/ash/wm/partial_screenshot_view.cc
index dd0bd1da0ac94148cada909d418ff7ac6b6692d5..a02ca7ef50405e91bc99bc7900b5912983566048 100644
--- a/ash/wm/partial_screenshot_view.cc
+++ b/ash/wm/partial_screenshot_view.cc
@@ -121,10 +121,9 @@ void PartialScreenshotView::OnMouseReleased(const ui::MouseEvent& event) {
   is_dragging_ = false;
   if (screenshot_delegate_) {
     aura::RootWindow *root_window = Shell::GetPrimaryRootWindow();
-    gfx::Rect root_window_screenshot_rect = root_window->bounds();
-    root_window_screenshot_rect.Intersect(GetScreenshotRect());
     screenshot_delegate_->HandleTakePartialScreenshot(
-        root_window, root_window_screenshot_rect);
+        root_window,
+        gfx::IntersectRects(root_window->bounds(), GetScreenshotRect()));
   }
 }