Add memory pressure dispatching callback to MemoryCoordinator

components/memory_coordinator/browser/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("browser") {
   sources = [
     "memory_coordinator.cc",
     "memory_coordinator.h",
   ]
 
   deps = [
     "//base",
     "//components/memory_coordinator/public/interfaces",
   ]
 
   public_deps = [
     "//components/memory_coordinator/common",
   ]
 }
+
+source_set("unit_tests") {
+  testonly = true
+
+  sources = [
+    "memory_coordinator_unittest.cc",
+  ]
+
+  deps = [
+    ":browser",
+    "//testing/gtest",
+  ]
+}