Remove base::mac::LibDispatchTaskRunner and migrate its sole client to plain libdispatch.

base/BUILD.gn

 # Copyright (c) 2013 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.
 
 # HOW TO WRITE CONDITIONALS IN THIS FILE
 # ======================================
 #
 # In many other places, one would write a conditional that expresses all the
 # cases when a source file is used or unused, and then either add or subtract
 # it from the sources list in that case
@@ skipping 388 matching lines @@
     "mac/close_nocancel.cc",
     "mac/cocoa_protocols.h",
     "mac/dispatch_source_mach.cc",
     "mac/dispatch_source_mach.h",
     "mac/foundation_util.h",
     "mac/foundation_util.mm",
     "mac/launch_services_util.cc",
     "mac/launch_services_util.h",
     "mac/launchd.cc",
     "mac/launchd.h",
-    "mac/libdispatch_task_runner.cc",
-    "mac/libdispatch_task_runner.h",
     "mac/mac_logging.h",
     "mac/mac_logging.mm",
     "mac/mac_util.h",
     "mac/mac_util.mm",
     "mac/mach_logging.cc",
     "mac/mach_logging.h",
     "mac/mach_port_broker.h",
     "mac/mach_port_broker.mm",
     "mac/mach_port_util.cc",
     "mac/mach_port_util.h",
@@ skipping 1365 matching lines @@
     "json/json_value_converter_unittest.cc",
     "json/json_value_serializer_unittest.cc",
     "json/json_writer_unittest.cc",
     "json/string_escape_unittest.cc",
     "lazy_instance_unittest.cc",
     "logging_unittest.cc",
     "mac/bind_objc_block_unittest.mm",
     "mac/call_with_eh_frame_unittest.mm",
     "mac/dispatch_source_mach_unittest.cc",
     "mac/foundation_util_unittest.mm",
-    "mac/libdispatch_task_runner_unittest.cc",
     "mac/mac_util_unittest.mm",
     "mac/mach_port_broker_unittest.cc",
     "mac/objc_property_releaser_unittest.mm",
     "mac/scoped_nsobject_unittest.mm",
     "mac/scoped_objc_class_swizzler_unittest.mm",
     "mac/scoped_sending_event_unittest.mm",
     "md5_unittest.cc",
     "memory/aligned_memory_unittest.cc",
     "memory/discardable_shared_memory_unittest.cc",
     "memory/linked_ptr_unittest.cc",
@@ skipping 612 matching lines @@
 
   # GYP: //base.gyp:base_java_unittest_support
   android_library("base_java_unittest_support") {
     deps = [
       ":base_java",
     ]
     java_files =
         [ "test/android/java/src/org/chromium/base/ContentUriTestUtils.java" ]
   }
 }