[fuchsia] Add tests for Fuchsia-specific layer behavior.

Inspect commands going to Scenic and make sure
they match what is expected.

Also, restructure code to need less member variables,
and other cleanups based on review feedback.

Author: mikejurka

flow/BUILD.gn

@@ -88,6 +88,7 @@ source_set("flow") {
 
     if (using_fuchsia_sdk) {
       public_deps += [
+        "$fuchsia_sdk_root/fidl:fuchsia.ui.app",
         "$fuchsia_sdk_root/fidl:fuchsia.ui.gfx",
         "$fuchsia_sdk_root/pkg:scenic_cpp",
       ]
@@ -159,6 +160,10 @@ executable("flow_unittests") {
     "texture_unittests.cc",
   ]
 
+  if (is_fuchsia) {
+    sources += ["layers/fuchsia_layer_unittests.cc",]
+  }
+
   deps = [
     ":flow",
     ":flow_fixtures",
@@ -170,6 +175,10 @@ executable("flow_unittests") {
     "//third_party/googletest:gtest",
     "//third_party/skia",
   ]
+
+  if (is_fuchsia) {
+    deps += ["//build/fuchsia/pkg:sys_cpp_testing",]
+  }
 }
 
 if (is_fuchsia) {

flow/layers/child_scene_layer.cc

@@ -51,7 +51,7 @@ void ChildSceneLayer::UpdateScene(SceneUpdateContext& context) {
   FML_DCHECK(view_holder);
 
   view_holder->UpdateScene(context, offset_, size_,
-                           sk_float_round2int(context.alphaf() * 255),
+                           SkScalarRoundToInt(context.alphaf() * 255),
                            hit_testable_);
 }