Add Mac memory pressure statistic reporting and consolidate platform code

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 1822 matching lines @@
     "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",
     "memory/memory_pressure_listener_unittest.cc",
     "memory/memory_pressure_monitor_chromeos_unittest.cc",
     "memory/memory_pressure_monitor_mac_unittest.cc",
+    "memory/memory_pressure_monitor_unittest.cc",
     "memory/memory_pressure_monitor_win_unittest.cc",
     "memory/ptr_util_unittest.cc",
     "memory/ref_counted_memory_unittest.cc",
     "memory/ref_counted_unittest.cc",
     "memory/scoped_vector_unittest.cc",
     "memory/shared_memory_mac_unittest.cc",
     "memory/shared_memory_unittest.cc",
     "memory/shared_memory_win_unittest.cc",
     "memory/singleton_unittest.cc",
     "memory/weak_ptr_unittest.cc",
@@ skipping 600 matching lines @@
     testonly = true
     deps = [
       ":base_java",
     ]
     java_files = [
       "test/android/java/src/org/chromium/base/ContentUriTestUtils.java",
       "test/android/java/src/org/chromium/base/TestSystemMessageHandler.java",
     ]
   }
 }