Hook up base unit tests and allocator to GN build.

build/config/BUILD.gn

Index: build/config/BUILD.gn
diff --git a/build/config/BUILD.gn b/build/config/BUILD.gn
index d83a221aa51a84ba8e4586cddaa676f4ea5ce134..48a33eda6f635af073a4bd5ba1553a12d3e216e6 100644
--- a/build/config/BUILD.gn
+++ b/build/config/BUILD.gn
@@ -2,6 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/config/allocator.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
@@ -120,6 +121,9 @@ config("feature_flags") {
       defines += [ "USE_XI2_MT=$use_xi2_mt" ]
     }
   }
+  if (use_allocator != "tcmalloc") {
+    defines += [ "NO_TCMALLOC" ]
+  }
 }
 
 # Debug/release ----------------------------------------------------------------