Add a unittest for image_data_fetcher

components/image_fetcher/BUILD.gn

 # Copyright 2016 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("image_fetcher") {
   sources = [
     "image_data_fetcher.cc",
     "image_data_fetcher.h",
     "image_decoder.h",
     "image_fetcher.h",
     "image_fetcher_delegate.h",
   ]
 
   public_deps = [
     "//base",
     "//net",
     "//url",
   ]
 }
+
+source_set("unit_tests") {
+  testonly = true
+  sources = [
+    "image_data_fetcher_unittest.cc",
+  ]
+  deps = [
+    ":image_fetcher",
+    "//net",
+    "//net:test_support",
+    "//testing/gmock",
+    "//testing/gtest",
+  ]
+}