Added MemoryMonitor for Linux (with test)

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.
 
 component("browser") {
   output_name = "memory_coordinator_browser"
 
   sources = [
     "memory_coordinator.cc",
     "memory_coordinator.h",
+    "memory_monitor.cc",
     "memory_monitor.h",
+    "memory_monitor_linux.cc",
+    "memory_monitor_linux.h",
     "memory_monitor_win.cc",
     "memory_monitor_win.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 = [
     "memory_coordinator_unittest.cc",
+    "memory_monitor_linux_unittest.cc",
     "memory_monitor_win_unittest.cc",
+    "test_memory_monitor.cc",
+    "test_memory_monitor.h",
   ]
 
   deps = [
     ":browser",
     "//testing/gtest",
   ]
 }