Let OverlayStrategySingleOnTop select non-topmost quad

cc/output/overlay_candidate.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/output/overlay_candidate.h"
 
 #include <algorithm>
 #include "base/logging.h"
 #include "ui/gfx/geometry/rect_conversions.h"
 
 namespace cc {
 
 OverlayCandidate::OverlayCandidate()
     : transform(gfx::OVERLAY_TRANSFORM_NONE),
       format(RGBA_8888),
       uv_rect(0.f, 0.f, 1.f, 1.f),
       resource_id(0),
       plane_z_order(0),
       overlay_handled(false) {}
 
 OverlayCandidate::~OverlayCandidate() {}
 
 // static
 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
                  : gfx::OVERLAY_TRANSFORM_NONE;
 }
 
 // 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);
   return gfx::ToNearestRect(float_rect);
 }
 
 }  // namespace cc