[bigint] API: Implement missing Big{Int,Uint}64Array::Cast

This doesn't change the API, it just makes it functional: the
functions were declared already, but they lacked an implementation
so far. Trying to use them in Blink detects that issue.

Bug: v8:6791
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I48902a5bba4a42f2922eafd22858d584731fc777
Reviewed-on: https://chromium-review.googlesource.com/1014668Reviewed-by: Daniel Clifford <danno@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52632}

include/v8.h

@@ -10166,6 +10166,19 @@ Float64Array* Float64Array::Cast(v8::Value* value) {
   return static_cast<Float64Array*>(value);
 }
 
+BigInt64Array* BigInt64Array::Cast(v8::Value* value) {
+#ifdef V8_ENABLE_CHECKS
+  CheckCast(value);
+#endif
+  return static_cast<BigInt64Array*>(value);
+}
+
+BigUint64Array* BigUint64Array::Cast(v8::Value* value) {
+#ifdef V8_ENABLE_CHECKS
+  CheckCast(value);
+#endif
+  return static_cast<BigUint64Array*>(value);
+}
 
 Uint8ClampedArray* Uint8ClampedArray::Cast(v8::Value* value) {
 #ifdef V8_ENABLE_CHECKS