Start refactoring Windows allocator shim in prep for hooking in generic allocator shim on Windows.

base/BUILD.gn

Index: base/BUILD.gn
diff --git a/base/BUILD.gn b/base/BUILD.gn
index be9f8a56e14a9434270ea961328ea3ace88b4a88..1391f3736cc32190f36b08ae87ebe62e8db7a554 100644
--- a/base/BUILD.gn
+++ b/base/BUILD.gn
@@ -1985,6 +1985,7 @@ test("base_unittests") {
     ":base",
     ":i18n",
     ":message_loop_tests",
+    "//base/allocator:features",

[Primiano Tucci (use gerrit), 2016/07/13 17:42:49]

Not sure this is required. tests depend on base which depends on
allocator:features. You should get this automatically by virtue of depending on
base. I suppose you are doing this not because it's required but more in the
spirit of Include What You Use.
No idea what is tha pettern in GN, some base owner will comment, I'm
indifferent.

[Sigurður Ásgeirsson, 2016/07/13 18:47:45]

It does seem it is.
Mmm, tests->base is a link-time dependency, you'd still want to be able to
compile all test and base file concurrently.
I actually get trybot errors without this, complaining that the OOM tests in
/process are including the results of a non-dependency. GN looks to be pretty
awesome that way :).
The trick to repro seems to

gn gen <output_dir> --check

     "//base/test:run_all_unittests",
     "//base/test:test_support",
     "//base/third_party/dynamic_annotations",