[Fuchsia] Grab the number of malloc'd bytes from jemalloc

runtime/BUILD.gn

Index: runtime/BUILD.gn
diff --git a/runtime/BUILD.gn b/runtime/BUILD.gn
index 852362a4df008277130464b18ccdd3e8edb5279e..29600f8036067a9176037ce695c869ed406d7cec 100644
--- a/runtime/BUILD.gn
+++ b/runtime/BUILD.gn
@@ -135,6 +135,11 @@ config("dart_config") {
     include_dirs += [ "../third_party/tcmalloc/gperftools/src" ]
   }
 
+  if (defined(is_fuchsia) && is_fuchsia) {
+    defines += [ "DART_USE_JEMALLOC" ]
+    include_dirs += [ "//magenta/third_party/ulib/jemalloc/include" ]
+  }
+
   if (!is_win) {
     cflags = [
       "-Werror",