[Favicon] Add new fallback icon rendering flow.

components/favicon_base/BUILD.gn

Index: components/favicon_base/BUILD.gn
diff --git a/components/favicon_base/BUILD.gn b/components/favicon_base/BUILD.gn
index 6e40b06718f9e7dad8a1a03aa53dd7474233afb0..b082acecfa22b4d290b35e07cb3bff6a2b03f19b 100644
--- a/components/favicon_base/BUILD.gn
+++ b/components/favicon_base/BUILD.gn
@@ -2,8 +2,12 @@
 # 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_specs.cc",
+    "fallback_icon_specs.h",
     "favicon_callback.h",
     "favicon_types.cc",
     "favicon_types.h",
@@ -20,3 +24,16 @@ source_set("favicon_base") {
     "//url",
   ]
 }
+
+source_set("unit_tests") {
+  sources = [
+    "fallback_icon_specs_unittest.cc",
+  ]
+
+  deps = [
+    ":favicon_base",
+    "//base",
+    "//base/test:test_support",
+    "//testing/gtest",
+  ]
+}