Add a new FORCE_DETERMINISTIC_BUILD define.

build/config/BUILD.gn

Index: build/config/BUILD.gn
diff --git a/build/config/BUILD.gn b/build/config/BUILD.gn
index 3fb9c9b1e855e2726ab3b36ea073f72cd0ec3fe8..e803ae3d10318a84d8ee588a99b17c01af0f256d 100644
--- a/build/config/BUILD.gn
+++ b/build/config/BUILD.gn
@@ -12,6 +12,12 @@ declare_args() {
   # that is normally turned on for Debug builds. These are generally useful for
   # catching bugs but in some cases may cause conflicts or excessive slowness.
   disable_iterator_debugging = false
+
+  # Set to true to force deterministic builds (this isn't working yet but this
+  # flag will help us to get there). See http://crbug.com/314403.
+  # TODO(sebmarchand): Update this comment once this flag guarantee a
+  #     deterministic build.
+  force_deterministic_build = false
 }
 
 # TODO(brettw) Most of these should be removed. Instead of global feature
@@ -71,6 +77,9 @@ config("feature_flags") {
   if (enable_spellcheck) {
     defines += [ "ENABLE_SPELLCHECK=1" ]
   }
+  if (force_deterministic_build) {
+    defines += [ "FORCE_DETERMINISTIC_BUILD" ]
+  }
   if (use_udev) {
     # TODO(brettw) should probably be "=1".
     defines += [ "USE_UDEV" ]