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

ash/display/multi_display_manager.cc

Index: ash/display/multi_display_manager.cc
diff --git a/ash/display/multi_display_manager.cc b/ash/display/multi_display_manager.cc
index 4f200eec92f6952da80085087243df9077630f8b..159e841de8a627c64e2310e9195c5a71fdad0e83 100644
--- a/ash/display/multi_display_manager.cc
+++ b/ash/display/multi_display_manager.cc
@@ -349,8 +349,7 @@ const gfx::Display& MultiDisplayManager::GetDisplayMatching(
   for (std::vector<gfx::Display>::const_iterator iter = displays_.begin();
        iter != displays_.end(); ++iter) {
     const gfx::Display& display = *iter;
-    gfx::Rect intersect = display.bounds();
-    intersect.Intersect(rect);
+    gfx::Rect intersect = gfx::Intersection(display.bounds(), rect);
     int area = intersect.width() * intersect.height();
     if (area > max) {
       max = area;