Initialize maths result array in JS.

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 cbf33a75d4c353aa98875caccd9bf9ce53549afb..70c587d7454b401fbfe9f2fc61812b039a6a0531 100644
--- a/src/runtime/runtime-maths.cc
+++ b/src/runtime/runtime-maths.cc
@@ -68,8 +68,8 @@ RUNTIME_FUNCTION(Runtime_RemPiO2) {
   CONVERT_DOUBLE_ARG_CHECKED(x, 0);
   CONVERT_ARG_CHECKED(JSTypedArray, result, 1);
   RUNTIME_ASSERT(result->byte_length() == Smi::FromInt(2 * sizeof(double)));
-  void* backing_store = JSArrayBuffer::cast(result->buffer())->backing_store();
-  double* y = static_cast<double*>(backing_store);
+  FixedFloat64Array* array = FixedFloat64Array::cast(result->elements());
+  double* y = static_cast<double*>(array->DataPtr());
   return Smi::FromInt(fdlibm::rempio2(x, y));
 }