Land prep work to enable NaCl in the Linux x64 GN builds.

base/third_party/dynamic_annotations/BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-source_set("dynamic_annotations") {
-  sources = [
-    "../valgrind/valgrind.h",
-    "dynamic_annotations.c",
-    "dynamic_annotations.h",
-  ]
-  if (is_android && !is_debug) {
-    configs -= [ "//build/config/compiler:optimize" ]
-    configs += [ "//build/config/compiler:optimize_max" ]
+if (is_nacl) {
+  # Native client doesn't need dynamic annotations, so we provide a
+  # dummy target in order for clients to not have to special-case the
+  # dependency.
+  group("dynamic_annotations") {
+  }
+} else {
+  source_set("dynamic_annotations") {
+    sources = [
+      "../valgrind/valgrind.h",
+      "dynamic_annotations.c",
+      "dynamic_annotations.h",
+    ]
+    if (is_android && !is_debug) {
+      configs -= [ "//build/config/compiler:optimize" ]
+      configs += [ "//build/config/compiler:optimize_max" ]
+    }
   }
 }