cc: Use to_screen transform for scroll offset snapping

cc/trees/layer_tree_host_unittest_scroll.cc

Index: cc/trees/layer_tree_host_unittest_scroll.cc
diff --git a/cc/trees/layer_tree_host_unittest_scroll.cc b/cc/trees/layer_tree_host_unittest_scroll.cc
index ddc5507a27cc4ae93c917f86121e17be8206a6b0..2a2d8448b7ed87921e08cc67365094e75d898c32 100644
--- a/cc/trees/layer_tree_host_unittest_scroll.cc
+++ b/cc/trees/layer_tree_host_unittest_scroll.cc
@@ -497,6 +497,64 @@ class LayerTreeHostScrollTestFractionalScroll : public LayerTreeHostScrollTest {
 
 MULTI_THREAD_TEST_F(LayerTreeHostScrollTestFractionalScroll);
 
+class LayerTreeHostScrollTestScrollSnapping : public LayerTreeHostScrollTest {
+ public:
+  LayerTreeHostScrollTestScrollSnapping() : scroll_amount_(1.75, 0) {}
+
+  void SetupTree() override {
+    LayerTreeHostScrollTest::SetupTree();
+    layer_tree_host()
+        ->outer_viewport_scroll_layer()
+        ->scroll_clip_layer()
+        ->SetForceRenderSurfaceForTesting(true);
+    gfx::Transform translate;
+    translate.Translate(0.25f, 0.f);
+    layer_tree_host()
+        ->outer_viewport_scroll_layer()
+        ->scroll_clip_layer()
+        ->SetTransform(translate);
+    layer_tree_host()->SetPageScaleFactorAndLimits(1.f, 0.1f, 100.f);
+  }
+
+  void BeginTest() override { PostSetNeedsCommitToMainThread(); }
+
+  void DrawLayersOnThread(LayerTreeHostImpl* impl) override {
+    LayerImpl* scroll_layer = impl->OuterViewportScrollLayer();
+    gfx::Transform translate;
+
+    // Check that screen space transform of the scrollable layer is correctly
+    // snapped to integers.
+    switch (impl->active_tree()->source_frame_number()) {
+      case 0:
+        translate.Translate(0, 0);
+        EXPECT_EQ(translate,

[sunxd, 2016/05/17 18:19:45]

I'm not sure if there is a better variable to compare here.

[ajuma, 2016/05/17 19:18:51]

You could compare to the identity (either by comparing to gfx::Transform(), or
by checking if screen_space_transform.IsIdentity is true).

[sunxd, 2016/05/17 19:38:01]

Done.

+                  scroll_layer->draw_properties().screen_space_transform);
+        PostSetNeedsCommitToMainThread();
+        break;
+      case 1:
+        translate.Translate(-2, 0);
+        EXPECT_EQ(translate,
+                  scroll_layer->draw_properties().screen_space_transform);
+        PostSetNeedsCommitToMainThread();
+        break;
+      case 2:
+        translate.Translate(-3, 0);
+        EXPECT_EQ(translate,
+                  scroll_layer->draw_properties().screen_space_transform);
+        EndTest();
+        break;
+    }
+    scroll_layer->ScrollBy(scroll_amount_);
+  }
+
+  void AfterTest() override {}
+
+ private:
+  gfx::Vector2dF scroll_amount_;
+};
+
+MULTI_THREAD_TEST_F(LayerTreeHostScrollTestScrollSnapping);
+
 class LayerTreeHostScrollTestCaseWithChild : public LayerTreeHostScrollTest {
  public:
   LayerTreeHostScrollTestCaseWithChild()