added fix that deals with precision in layer sorter

cc/layer_sorter_unittest.cc

 // Copyright 2011 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 "config.h"
 
 #include "cc/layer_sorter.h"
 
 #include "cc/layer_impl.h"
 #include "cc/math_util.h"
@@ skipping 246 matching lines @@
     layerSorter.sort(layerList.begin(), layerList.end());
 
     ASSERT_EQ(static_cast<size_t>(5), layerList.size());
     EXPECT_EQ(3, layerList[0]->id());
     EXPECT_EQ(4, layerList[1]->id());
     EXPECT_EQ(1, layerList[2]->id());
     EXPECT_EQ(2, layerList[3]->id());
     EXPECT_EQ(5, layerList[4]->id());
 }
 
+TEST(LayerSorterTest, verifyConcidentLayerPrecisionLossResultsInDocumentOrder)
+{
+    DebugScopedSetImplThread thisScopeIsOnImplThread;
+
+    scoped_ptr<LayerImpl> layer1 = LayerImpl::create(1);
+    scoped_ptr<LayerImpl> layer2 = LayerImpl::create(2);
+
+    WebTransformationMatrix BehindMatrix;
+    BehindMatrix.translate3d(0, 0, 0.999999f);
+    BehindMatrix.rotate3d(38.5f, 77.0f, 0);
+    WebTransformationMatrix FrontMatrix;
+    FrontMatrix.translate3d(0, 0, 1.0f);
+    FrontMatrix.rotate3d(38.5f, 77.0f, 0);
+
+    layer1->setBounds(IntSize(10, 10));
+    layer1->setContentBounds(IntSize(10, 10));
+    layer1->setDrawTransform(BehindMatrix);
+    layer1->setDrawsContent(true);
+
+    layer2->setBounds(IntSize(10, 10));
+    layer2->setContentBounds(IntSize(10, 10));
+    layer2->setDrawTransform(FrontMatrix);
+    layer2->setDrawsContent(true);
+
+    std::vector<LayerImpl*> layerList;
+    layerList.push_back(layer1.get());

[enne (OOO), 2012/11/01 20:08:55]

Does this test pass before this patch? I thought positive Z was out of the page,
so painter's algorithm would paint lower Z values before higher ones.

+    layerList.push_back(layer2.get());
+
+    ASSERT_EQ(static_cast<size_t>(2), layerList.size());
+    EXPECT_EQ(1, layerList[0]->id());
+    EXPECT_EQ(2, layerList[1]->id());
+
+    LayerSorter layerSorter;
+    layerSorter.sort(layerList.begin(), layerList.end());
+
+    ASSERT_EQ(static_cast<size_t>(2), layerList.size());
+    EXPECT_EQ(1, layerList[0]->id());
+    EXPECT_EQ(2, layerList[1]->id());
+    EXPECT_FALSE(1.0f == 999999f);

[enne (OOO), 2012/11/01 20:08:55]

Missing a period here. Also, maybe better to actually just get the value out of
layer1->drawTransform().m43() than to repeat the literals?

+}
+
 } // namespace