Add tcmalloc to GN build

runtime/bin/BUILD.gn

Index: runtime/bin/BUILD.gn
diff --git a/runtime/bin/BUILD.gn b/runtime/bin/BUILD.gn
index f09dd415b9a0f99179020d1159c84b27cb1c27a5..549807b8ff385da0cd227808c59288c9b9740ad6 100644
--- a/runtime/bin/BUILD.gn
+++ b/runtime/bin/BUILD.gn
@@ -12,6 +12,10 @@ declare_args() {
   # VM, we set this to //runtime/bin/zlib where we have a BUILD.gn file without
   # a dependence on //base.
   dart_zlib_path = "//third_party/zlib"
+
+  # Whether to link the standalone VM against tcmalloc. The standalone build of
+  # the VM enables this only for Linux builds.
+  dart_use_tcmalloc = false
 }
 
 resources_sources_gypi =
@@ -516,6 +520,12 @@ template("dart_executable") {
       "$dart_zlib_path",
     ] + extra_deps
 
+    if (dart_use_tcmalloc) {
+      deps += [
+        "//third_party/tcmalloc",
+      ]
+    }
+
     defines = extra_defines
 
     sources = [
@@ -665,6 +675,12 @@ executable("run_vm_tests") {
     "TESTING",
   ]
 
+  if (dart_use_tcmalloc) {
+    deps += [
+      "//third_party/tcmalloc",
+    ]
+  }
+
   # The VM sources are already included in libdart, so we just want to add in
   # the tests here.
   vm_tests_list = exec_script("../../tools/gypi_to_gn.py",