[bigint] Make ToString's radix argument optional with default 10.

R=jkummerow@chromium.org

Bug: v8:6791
Change-Id: I7b3efcd0033ecb8c872342cd573f416fd22daf73
Reviewed-on: https://chromium-review.googlesource.com/707006Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48402}

src/objects.cc

@@ -255,7 +255,7 @@ MaybeHandle<String> Object::ConvertToString(Isolate* isolate,
                       String);
     }
     if (input->IsBigInt()) {
-      return BigInt::ToString(Handle<BigInt>::cast(input), 10);
+      return BigInt::ToString(Handle<BigInt>::cast(input));
     }
     ASSIGN_RETURN_ON_EXCEPTION(
         isolate, input, JSReceiver::ToPrimitive(Handle<JSReceiver>::cast(input),

src/objects/bigint.h

@@ -62,7 +62,7 @@ class BigInt : public HeapObject {
   }
   void Initialize(int length, bool zero_initialize);
 
-  static MaybeHandle<String> ToString(Handle<BigInt> bigint, int radix);
+  static MaybeHandle<String> ToString(Handle<BigInt> bigint, int radix = 10);
 
   // Temporarily exposed helper, pending proper initialization.
   void set_value(int value) {