Move ImageFetcher to its own target.

ios/chrome/browser/webp_transcode/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("webp_transcode") {
-  sources = [
-    "webp_decoder.h",
-    "webp_decoder.mm",
-  ]
-  deps = [
-    "//base",
-    "//third_party/libwebp:libwebp_dec",
-  ]
-}
-
-source_set("unit_tests") {
-  testonly = true
-  sources = [
-    "webp_decoder_unittest.mm",
-  ]
-  deps = [
-    ":webp_transcode",
-    ":webp_transcode_unit_tests_bundle_data",
-    "//base",
-    "//testing/gmock",
-    "//testing/gtest",
-  ]
-  libs = [ "CoreGraphics.framework" ]
-}
-
-bundle_data("webp_transcode_unit_tests_bundle_data") {
-  visibility = [ ":unit_tests" ]
-  testonly = true
-  sources = [
-    "//ios/chrome/test/data/webp_transcode/test.jpg",
-    "//ios/chrome/test/data/webp_transcode/test.webp",
-    "//ios/chrome/test/data/webp_transcode/test_alpha.png",
-    "//ios/chrome/test/data/webp_transcode/test_alpha.webp",
-    "//ios/chrome/test/data/webp_transcode/test_small.tiff",
-    "//ios/chrome/test/data/webp_transcode/test_small.webp",
-  ]
-  outputs = [
-    "{{bundle_resources_dir}}/{{source_root_relative_dir}}/" +
-        "{{source_file_part}}",
-  ]
-}