Ensure external snapshot is only set once.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index c54cdd21a38e8b700c2c9b6df140a4772e3b02eb..70758791fa4cfb751037894425cbe5d32df1f207 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -325,20 +325,12 @@ void V8::SetDecompressedStartupData(StartupData* decompressed_data) {
 
 
 void V8::SetNativesDataBlob(StartupData* natives_blob) {
-#ifdef V8_USE_EXTERNAL_STARTUP_DATA
-  i::SetNativesFromFile(natives_blob);
-#else
-  CHECK(false);
-#endif
+  i::V8::SetNativesBlob(natives_blob);
 }
 
 
 void V8::SetSnapshotDataBlob(StartupData* snapshot_blob) {
-#ifdef V8_USE_EXTERNAL_STARTUP_DATA
-  i::SetSnapshotFromFile(snapshot_blob);
-#else
-  CHECK(false);
-#endif
+  i::V8::SetSnapshotBlob(snapshot_blob);
 }