memory_coordinator: Use component(target) instead of static_library(target)

components/memory_coordinator/child/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.
 
-static_library("child") {
+component("child") {
+  output_name = "memory_coordinator_child"
+
   sources = [
     "child_memory_coordinator_impl.cc",
     "child_memory_coordinator_impl.h",
   ]
 
+  defines = [ "MEMORY_COORDINATOR_IMPLEMENTATION" ]
+
   deps = [
     "//base",
     "//components/memory_coordinator/public/interfaces",
   ]
 
   public_deps = [
     "//components/memory_coordinator/common",
   ]
 }
 
 source_set("unit_tests") {
   testonly = true
 
   sources = [
     "child_memory_coordinator_impl_unittest.cc",
   ]
 
   deps = [
     ":child",
     "//mojo/common",
     "//testing/gtest",
   ]
 }