Ignore rounding error between clip_layer_length_ and scroll_layer_length_

cc/layers/solid_color_scrollbar_layer_impl_unittest.cc

Index: cc/layers/solid_color_scrollbar_layer_impl_unittest.cc
diff --git a/cc/layers/solid_color_scrollbar_layer_impl_unittest.cc b/cc/layers/solid_color_scrollbar_layer_impl_unittest.cc
index a2b24303f235538035b4ab0095d4300c8ebe3f4d..f1565731097d23edaa3e4341a89744a39ed38545 100644
--- a/cc/layers/solid_color_scrollbar_layer_impl_unittest.cc
+++ b/cc/layers/solid_color_scrollbar_layer_impl_unittest.cc
@@ -77,5 +77,47 @@ TEST(SolidColorScrollbarLayerImplTest, Occlusion) {
   }
 }
 
+TEST(SolidColorScrollbarLayerImplTest, RoundingError) {

[bokan, 2017/04/18 17:39:08]

This isn't specific to solid color scrollbar layers, lets put it in
scrollbar_layer_unittest or somewhere more general.

Also, please use a more descriptive name for the test. Perhaps
SubPixelCanScrollOrientation

+  gfx::Size viewport_size(100, 100);
+
+  LayerTestCommon::LayerImplTest impl;
+
+  LayerImpl* clip_layer = impl.AddChildToRoot<LayerImpl>();
+  LayerImpl* scroll_layer = impl.AddChild<LayerImpl>(clip_layer);
+
+  scroll_layer->SetScrollClipLayer(clip_layer->id());
+
+  const int kTrackStart = 0;
+  const int kThumbThickness = 10;
+  const bool kIsLeftSideVerticalScrollbar = false;
+  const bool kIsOverlayScrollbar = false;
+
+  SolidColorScrollbarLayerImpl* scrollbar_layer =
+      impl.AddChild<SolidColorScrollbarLayerImpl>(
+          scroll_layer, HORIZONTAL, kThumbThickness, kTrackStart,
+          kIsLeftSideVerticalScrollbar, kIsOverlayScrollbar);
+
+  scrollbar_layer->SetScrollLayerId(scroll_layer->id());
+  clip_layer->SetBounds(gfx::Size(100, 100));
+  scroll_layer->SetBounds(gfx::Size(100, 100));
+
+  impl.CalcDrawProps(viewport_size);
+
+  EXPECT_FLOAT_EQ(100.0f, scrollbar_layer->clip_layer_length());
+  EXPECT_FLOAT_EQ(100.0f, scrollbar_layer->scroll_layer_length());

[bokan, 2017/04/18 17:39:08]

No need to test these.

+
+  // Fake clip layer length to scrollbar to mock rounding error.
+  scrollbar_layer->SetClipLayerLength(99.99f);
+  // Fake device scale.
+  impl.host_impl()->active_tree()->SetDeviceScaleFactor(3);
+
+  EXPECT_FLOAT_EQ(99.99f, scrollbar_layer->clip_layer_length());
+  EXPECT_FLOAT_EQ(3.f, impl.host_impl()->active_tree()->device_scale_factor());
+  EXPECT_FLOAT_EQ(1.f,
+                  impl.host_impl()->active_tree()->current_page_scale_factor());

[bokan, 2017/04/18 17:39:08]

No need to test any of the above lines. If you really want a "sanity-check", you
should ASSERT rather than EXPECT. A failing ASSERT means the test itself is
broken, failing EXPECT means what we're checking is broken.

+
+  EXPECT_FALSE(scrollbar_layer->CanScrollOrientation());

[bokan, 2017/04/18 17:39:08]

You should also test the converse, that if the clip layer has a device pixel of
difference from the scroll layer it is scrollable. 

+}
+
 }  // namespace
 }  // namespace cc