Update jpeg writer to compile on iOS and document it better

webrtc/test/BUILD.gn

 # Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 import("../webrtc.gni")
 import("//build/config/ui.gni")
@@ skipping 208 matching lines @@
       "../rtc_base:rtc_base_approved",
       "../system_wrappers",
       "//testing/gmock",
       "//testing/gtest",
       "//third_party/gflags",
     ]
 
     if (!is_ios) {
       deps += [ "//third_party:jpeg" ]
       sources += [ "testsupport/jpeg_frame_writer.cc" ]
+    } else {
+      sources += [ "testsupport/jpeg_frame_writer_ios.cc" ]
     }
 
     public_deps = [
       ":fileutils",
     ]
 
     if (!build_with_chromium && is_clang) {
       # Suppress warnings from the Chromium Clang plugin (bugs.webrtc.org/163).
       suppressed_configs += [ "//build/config/clang:find_bad_constructs" ]
     }
@@ skipping 476 matching lines @@
     ]
     deps = [
       "../rtc_base:base_java",
       "//testing/android/native_test:native_test_java",
     ]
 
     # TODO(sakal): Fix build hooks crbug.com/webrtc/8148
     no_build_hooks = true
   }
 }