Add an external_startup_data config to the GN buildfile.

This change defines a common config for clients of v8 that
need to define V8_USE_EXTERNAL_STARTUP_DATA in order for
client code to be compiled correctly.

R=brettw@chromium.org, jochen@chromium.org
BUG=541791
LOG=n

Review URL: https://codereview.chromium.org/1415773003

Cr-Commit-Position: refs/heads/master@{#31427}

BUILD.gn

@@ -71,6 +71,14 @@ config("external_config") {
   include_dirs = [ "include" ]
 }
 
+# This config should only be applied to code that needs to be explicitly
+# aware of whether we are using startup data or not.
+config("external_startup_data") {
+  if (v8_use_external_startup_data) {
+    defines = [ "V8_USE_EXTERNAL_STARTUP_DATA" ]
+  }
+}
+
 config("features") {
   visibility = [ ":*" ]  # Only targets in this file can depend on this.