Let SetEntropySource() fail if called after V8::Initialize().

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index 4206105424d4ed73be4e67c071e810f68d3f57f3..28110826ba3cf10d5ad2ac8c20d29cd5bf7eb325 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -4540,6 +4540,9 @@ class V8_EXPORT V8 {
   /**
    * Allows the host application to provide a callback which can be used
    * as a source of entropy for random number generators.
+   *
+   * \note Setting an entropy source can only be done very early prior to
+   *   calling Initialize().
    */
   static void SetEntropySource(EntropySource source);