Let OverlayStrategySingleOnTop select non-topmost quad

cc/output/overlay_candidate.cc

Index: cc/output/overlay_candidate.cc
diff --git a/cc/output/overlay_candidate.cc b/cc/output/overlay_candidate.cc
index 41682539a58223201db0063681e5d43a6d7f043d..307bb22359e213487022f202ad5809fb316fc32c 100644
--- a/cc/output/overlay_candidate.cc
+++ b/cc/output/overlay_candidate.cc
@@ -24,7 +24,7 @@ OverlayCandidate::~OverlayCandidate() {}
 gfx::OverlayTransform OverlayCandidate::GetOverlayTransform(
     const gfx::Transform& quad_transform,
     bool flipped) {
-  if (!quad_transform.IsIdentityOrTranslation())
+  if (!quad_transform.IsPositiveScaleOrTranslation())
     return gfx::OVERLAY_TRANSFORM_INVALID;
 
   return flipped ? gfx::OVERLAY_TRANSFORM_FLIP_VERTICAL
@@ -34,7 +34,7 @@ gfx::OverlayTransform OverlayCandidate::GetOverlayTransform(
 // static
 gfx::Rect OverlayCandidate::GetOverlayRect(const gfx::Transform& quad_transform,
                                            const gfx::Rect& rect) {
-  DCHECK(quad_transform.IsIdentityOrTranslation());
+  DCHECK(quad_transform.IsPositiveScaleOrTranslation());
 
   gfx::RectF float_rect(rect);
   quad_transform.TransformRect(&float_rect);