base: Introduce SharedMemoryTracker for POSIX (but not macOS)

base/BUILD.gn

Index: base/BUILD.gn
diff --git a/base/BUILD.gn b/base/BUILD.gn
index fc39018efc63064b454280af10a0892817190c72..c6a8b626a8aea5daa235b5099828e96f3969b623 100644
--- a/base/BUILD.gn
+++ b/base/BUILD.gn
@@ -1577,6 +1577,13 @@ component("base") {
     sources += [ "power_monitor/power_monitor_device_source_posix.cc" ]
   }
 
+  if (is_posix && !is_mac && !is_nacl) {
+    sources += [
+      "memory/shared_memory_tracker.cc",
+      "memory/shared_memory_tracker.h",
+    ]
+  }
+
   if (!use_glib) {
     sources -= [
       "message_loop/message_pump_glib.cc",