Added MemoryMonitor for Linux (with test)

components/memory_coordinator/browser/BUILD.gn

Index: components/memory_coordinator/browser/BUILD.gn
diff --git a/components/memory_coordinator/browser/BUILD.gn b/components/memory_coordinator/browser/BUILD.gn
index c4ac3657b47042d98c9c7c21770e0f0a5025b4cb..834a26f8bb5f3a372ff2256d3200b70d156c080d 100644
--- a/components/memory_coordinator/browser/BUILD.gn
+++ b/components/memory_coordinator/browser/BUILD.gn
@@ -8,7 +8,10 @@ component("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",
   ]
@@ -30,7 +33,10 @@ source_set("unit_tests") {
 
   sources = [
     "memory_coordinator_unittest.cc",
+    "memory_monitor_linux_unittest.cc",
     "memory_monitor_win_unittest.cc",
+    "test_memory_monitor.cc",
+    "test_memory_monitor.h",
   ]
 
   deps = [