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 d93321b3e1f26549895bc0f222aa3048ed403eb4..5911ae92a06d1bf2cdbd1ec03632f510750ba221 100644
--- a/ash/display/multi_display_manager.cc
+++ b/ash/display/multi_display_manager.cc
@@ -360,8 +360,7 @@ 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::IntersectRects(display.bounds(), rect);
     int area = intersect.width() * intersect.height();
     if (area > max) {
       max = area;