Add a Model to the download component.

components/download/internal/BUILD.gn

Index: components/download/internal/BUILD.gn
diff --git a/components/download/internal/BUILD.gn b/components/download/internal/BUILD.gn
index a584dc5463d669015b17bc1609606562cff71f3d..99a2a67d2c4de341064ee2d287d3d7f26df21cc8 100644
--- a/components/download/internal/BUILD.gn
+++ b/components/download/internal/BUILD.gn
@@ -11,11 +11,20 @@ static_library("internal") {
   visibility = [
     ":*",
     "//components/download",
+    "//components/download/internal/test_support",
   ]
 
   sources = [
     "download_service_impl.cc",
     "download_service_impl.h",
+    "entry.cc",
+    "entry.h",
+    "model.h",
+    "model_impl.cc",
+    "model_impl.h",
+    "noop_store.cc",
+    "noop_store.h",
+    "store.h",
   ]
 
   deps = [
@@ -30,12 +39,13 @@ source_set("unit_tests") {
   visibility = [ "//components/download:unit_tests" ]
 
   sources = [
-    "download_service_impl_unittest.cc",
+    "model_impl_unittest.cc",
   ]
 
   deps = [
     ":internal",
     "//base/test:test_support",
+    "//components/download/internal/test_support",
     "//testing/gtest",
   ]
 }