Add memory pressure monitor for Linux.

components/memory_pressure/BUILD.gn

 # Copyright 2015 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.
 
 # GYP version: components/memory_pressure.gypi:memory_pressure
 source_set("memory_pressure") {
   sources = [
     "direct_memory_pressure_calculator.h",
+    "direct_memory_pressure_calculator_linux.cc",
+    "direct_memory_pressure_calculator_linux.h",
     "direct_memory_pressure_calculator_win.cc",
     "direct_memory_pressure_calculator_win.h",
     "filtered_memory_pressure_calculator.cc",
     "filtered_memory_pressure_calculator.h",
     "memory_pressure_calculator.h",
     "memory_pressure_listener.cc",
     "memory_pressure_listener.h",
     "memory_pressure_stats_collector.cc",
     "memory_pressure_stats_collector.h",
   ]
 
   deps = [
     "//base",
   ]
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
+    "direct_memory_pressure_calculator_linux_unittest.cc",
     "direct_memory_pressure_calculator_win_unittest.cc",
     "filtered_memory_pressure_calculator_unittest.cc",
     "memory_pressure_stats_collector_unittest.cc",
     "test_memory_pressure_calculator.cc",
     "test_memory_pressure_calculator.h",
   ]
 
   deps = [
     ":memory_pressure",
     "//base/test:test_support",
     "//testing/gtest",
   ]
 }