Move ImageFetcher to its own target.

ios/chrome/browser/net/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_fetcher.h",
+    "image_fetcher.mm",
+  ]
+  deps = [
+    "//base",
+    "//ios/chrome/browser/webp_transcode",
+    "//ios/web",
+    "//net",
+  ]
+}
+
+source_set("test_support") {
+  testonly = true
+  sources = [
+    "mock_image_fetcher.h",
+    "mock_image_fetcher.mm",
+  ]
+  deps = [
+    ":image_fetcher",
+    "//testing/gmock",
+  ]
+}
+
+source_set("unit_tests") {
+  testonly = true
+  sources = [
+    "image_fetcher_unittest.mm",
+  ]
+  deps = [
+    ":image_fetcher",
+    "//base",
+    "//net",
+    "//net:test_support",
+    "//testing/gtest",
+  ]
+}