cc: Make PictureLayerImpl use a better choice for animated raster scale.

cc/layers/picture_layer_impl_unittest.cc

Index: cc/layers/picture_layer_impl_unittest.cc
diff --git a/cc/layers/picture_layer_impl_unittest.cc b/cc/layers/picture_layer_impl_unittest.cc
index 3ceb79bc34acf91037a3d69526181b8eeae1d25d..11a84b0e7a848f4a35b75b0c7a1643f55f9ef0bc 100644
--- a/cc/layers/picture_layer_impl_unittest.cc
+++ b/cc/layers/picture_layer_impl_unittest.cc
@@ -1095,6 +1095,7 @@ TEST_F(PictureLayerImplTest, DontAddLowResDuringAnimation) {
   // Page scale animation, new high res, but not new low res because animating.
   contents_scale = 2.f;
   page_scale = 2.f;
+  maximum_animation_scale = 2.f;
   animating_transform = true;
   SetContentsScaleOnBothLayers(contents_scale,
                                device_scale,
@@ -2380,7 +2381,7 @@ TEST_F(PictureLayerImplTest, HighResTilingDuringAnimationForCpuRasterization) {
   EXPECT_BOTH_EQ(HighResTiling()->contents_scale(), 4.f);
 
   // When animating with an unknown maximum animation scale factor, a new
-  // high-res tiling should be created at the animation's initial scale.
+  // high-res tiling should be created at a source scale of 1.
   animating_transform = true;
   contents_scale = 2.f;
   maximum_animation_scale = 0.f;
@@ -2390,7 +2391,7 @@ TEST_F(PictureLayerImplTest, HighResTilingDuringAnimationForCpuRasterization) {
                                page_scale,
                                maximum_animation_scale,
                                animating_transform);
-  EXPECT_BOTH_EQ(HighResTiling()->contents_scale(), 2.f);
+  EXPECT_BOTH_EQ(HighResTiling()->contents_scale(), page_scale * device_scale);
 
   // Further changes to scale during the animation should not cause a new
   // high-res tiling to get created.
@@ -2401,7 +2402,7 @@ TEST_F(PictureLayerImplTest, HighResTilingDuringAnimationForCpuRasterization) {
                                page_scale,
                                maximum_animation_scale,
                                animating_transform);
-  EXPECT_BOTH_EQ(HighResTiling()->contents_scale(), 2.f);
+  EXPECT_BOTH_EQ(HighResTiling()->contents_scale(), page_scale * device_scale);
 
   // Once we stop animating, a new high-res tiling should be created.
   animating_transform = false;
@@ -2416,8 +2417,8 @@ TEST_F(PictureLayerImplTest, HighResTilingDuringAnimationForCpuRasterization) {
 
   // When animating with a maxmium animation scale factor that is so large
   // that the layer grows larger than the viewport at this scale, a new
-  // high-res tiling should get created at the animation's initial scale, not
-  // at its maximum scale.
+  // high-res tiling should get created at a source scale of 1, not at its
+  // maximum scale.
   animating_transform = true;
   contents_scale = 2.f;
   maximum_animation_scale = 11.f;
@@ -2427,7 +2428,7 @@ TEST_F(PictureLayerImplTest, HighResTilingDuringAnimationForCpuRasterization) {
                                page_scale,
                                maximum_animation_scale,
                                animating_transform);
-  EXPECT_BOTH_EQ(HighResTiling()->contents_scale(), 2.f);
+  EXPECT_BOTH_EQ(HighResTiling()->contents_scale(), page_scale * device_scale);
 
   // Once we stop animating, a new high-res tiling should be created.
   animating_transform = false;
@@ -2457,6 +2458,31 @@ TEST_F(PictureLayerImplTest, HighResTilingDuringAnimationForCpuRasterization) {
 
   // Once we stop animating, a new high-res tiling should be created.
   animating_transform = false;
+  contents_scale = 12.f;
+
+  SetContentsScaleOnBothLayers(contents_scale,
+                               device_scale,
+                               page_scale,
+                               maximum_animation_scale,
+                               animating_transform);
+  EXPECT_BOTH_EQ(HighResTiling()->contents_scale(), 12.f);
+
+  // When animating toward a smaller scale, but that is still so large that the
+  // layer grows larger than the viewport at this scale, a new high-res tiling
+  // should get created at source scale 1.
+  animating_transform = true;
+  contents_scale = 11.f;
+  maximum_animation_scale = 11.f;
+
+  SetContentsScaleOnBothLayers(contents_scale,
+                               device_scale,
+                               page_scale,
+                               maximum_animation_scale,
+                               animating_transform);
+  EXPECT_BOTH_EQ(HighResTiling()->contents_scale(), device_scale * page_scale);
+
+  // Once we stop animating, a new high-res tiling should be created.
+  animating_transform = false;
   contents_scale = 11.f;
 
   SetContentsScaleOnBothLayers(contents_scale,