Move iOS image_fetcher::ImageDecoder to layered component

ios/chrome/browser/suggestions/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("suggestions") {
   configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
-    "ios_image_decoder_impl.h",
-    "ios_image_decoder_impl.mm",
     "suggestions_service_factory.h",
     "suggestions_service_factory.mm",
   ]
   deps = [
     "//base",
     "//components/browser_sync",
     "//components/image_fetcher",
     "//components/image_fetcher/ios",
     "//components/keyed_service/ios",
     "//components/leveldb_proto",
     "//components/signin/core/browser",
     "//components/suggestions",
     "//ios/chrome/browser/browser_state",
     "//ios/chrome/browser/signin",
     "//ios/chrome/browser/sync",
     "//ios/web",
     "//net",
     "//skia",
     "//ui/gfx",
   ]
 }
-
-source_set("unit_tests") {
-  configs += [ "//build/config/compiler:enable_arc" ]
-  testonly = true
-  sources = [
-    "ios_image_decoder_impl_unittest.mm",
-  ]
-  deps = [
-    ":suggestions",
-    "//base",
-    "//testing/gtest",
-    "//ui/gfx",
-  ]
-}