Bring in a reduced/simplified version of Chromium's base::WeakPtr to //mojo/edk/util.

mojo/edk/util/BUILD.gn

Index: mojo/edk/util/BUILD.gn
diff --git a/mojo/edk/util/BUILD.gn b/mojo/edk/util/BUILD.gn
index 371c29287b01c662e664fc4a3580e04f3a82cd04..20241946c7694c54d74c81bb606e30b3c5141bfb 100644
--- a/mojo/edk/util/BUILD.gn
+++ b/mojo/edk/util/BUILD.gn
@@ -32,6 +32,9 @@ mojo_edk_source_set("util") {
     "thread_checker.h",
     "waitable_event.cc",
     "waitable_event.h",
+    "weak_ptr.h",
+    "weak_ptr_internal.cc",
+    "weak_ptr_internal.h",
   ]
 
   mojo_sdk_public_deps = [ "mojo/public/cpp/system" ]
@@ -54,6 +57,7 @@ mojo_edk_source_set("unittests") {
     "thread_annotations_unittest.cc",
     "thread_checker_unittest.cc",
     "waitable_event_unittest.cc",
+    "weak_ptr_unittest.cc",
   ]
 
   deps = [