[Favicon] Add new fallback icon rendering flow.

components/favicon_base/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-source_set("favicon_base") {
+static_library("favicon_base") {
   sources = [
+    "fallback_icon_service.cc",
+    "fallback_icon_service.h",
+    "fallback_icon_style.cc",
+    "fallback_icon_style.h",
     "favicon_callback.h",
     "favicon_types.cc",
     "favicon_types.h",
     "favicon_util.cc",
     "favicon_util.h",
     "select_favicon_frames.cc",
     "select_favicon_frames.h",
   ]
 
   deps = [
     "//base",
     "//skia",
     "//ui/gfx",
     "//url",
   ]
 }
+
+source_set("unit_tests") {
+  sources = [
+    "fallback_icon_style_unittest.cc",
+  ]
+
+  deps = [
+    ":favicon_base",
+    "//base",
+    "//base/test:test_support",
+    "//testing/gtest",
+  ]
+}