Allow Math.random to be called when creating a custom startup snapshot.

src/runtime/runtime-maths.cc

Index: src/runtime/runtime-maths.cc
diff --git a/src/runtime/runtime-maths.cc b/src/runtime/runtime-maths.cc
index 59bf372b00a8bab3becb3c05c6111285c98617cf..8a43c2758eb2c0051a3b3be7dbdb990ab57ef24b 100644
--- a/src/runtime/runtime-maths.cc
+++ b/src/runtime/runtime-maths.cc
@@ -228,8 +228,18 @@ RUNTIME_FUNCTION(Runtime_MathSqrt) {
 RUNTIME_FUNCTION(Runtime_GenerateRandomNumbers) {
   HandleScope scope(isolate);
   DCHECK(args.length() == 1);
-  // Random numbers in the snapshot are not really that random.
-  CHECK(!isolate->serializer_enabled());
+  if (isolate->serializer_enabled()) {
+    // Random numbers in the snapshot are not really that random. And we cannot
+    // return a typed array as it cannot be serialized. To make calling
+    // Math.random possible when creating a custom startup snapshot, we simply
+    // return a normal array with a single random number.
+    Handle<HeapNumber> random_number = isolate->factory()->NewHeapNumber(
+        isolate->random_number_generator()->NextDouble());
+    Handle<FixedArray> array_backing = isolate->factory()->NewFixedArray(1);
+    array_backing->set(0, *random_number);
+    return *isolate->factory()->NewJSArrayWithElements(array_backing);
+  }
+
   static const int kState0Offset = 0;
   static const int kState1Offset = 1;
   static const int kRandomBatchSize = 64;