cc: Don't clear transparent piles to an opaque color

cc/layers/layer_impl_unittest.cc

Index: cc/layers/layer_impl_unittest.cc
diff --git a/cc/layers/layer_impl_unittest.cc b/cc/layers/layer_impl_unittest.cc
index 01dda88ada7f96c3602e6f46fad00bf1c7bc7cbe..dc1b29bc9ed47309745b6afe6a81429edb9c6d72 100644
--- a/cc/layers/layer_impl_unittest.cc
+++ b/cc/layers/layer_impl_unittest.cc
@@ -293,5 +293,35 @@ TEST(LayerImplTest, VerifyNeedsUpdateDrawProperties) {
   VERIFY_NO_NEEDS_UPDATE_DRAW_PROPERTIES(root->SetBounds(arbitrary_size));
 }
 
+TEST(LayerImplTest, SafeOpaqueBackgroundColor) {
+  FakeImplProxy proxy;
+  FakeLayerTreeHostImpl host_impl(&proxy);
+  EXPECT_TRUE(host_impl.InitializeRenderer(CreateFakeOutputSurface()));
+  scoped_ptr<LayerImpl> layer = LayerImpl::Create(host_impl.active_tree(), 1);
+
+  for (int contents_opaque = 0; contents_opaque < 2; ++contents_opaque) {
+    for (int layer_opaque = 0; layer_opaque < 2; ++layer_opaque) {
+      for (int host_opaque = 0; host_opaque < 2; ++host_opaque) {
+        layer->SetContentsOpaque(!!contents_opaque);
+        layer->SetBackgroundColor(layer_opaque ? SK_ColorRED
+                                               : SK_ColorTRANSPARENT);
+        host_impl.active_tree()->set_background_color(
+            host_opaque ? SK_ColorRED : SK_ColorTRANSPARENT);
+
+        SkColor safe_color = layer->SafeOpaqueBackgroundColor();
+        if (contents_opaque) {
+          EXPECT_EQ(SkColorGetA(safe_color), 255u)
+              << "Flags: " << contents_opaque << ", " << layer_opaque << ", "
+              << host_opaque << "\n";
+        } else {
+          EXPECT_NE(SkColorGetA(safe_color), 255u)
+              << "Flags: " << contents_opaque << ", " << layer_opaque << ", "
+              << host_opaque << "\n";
+        }
+      }
+    }
+  }
+}
+
 }  // namespace
 }  // namespace cc