[bigint] Flatten strings in StringToBigInt

Turns out that all callers might need this, so instead of burdening them
with checking, make the helper more robust.

Bug: v8:7151
Change-Id: I54270e2b1f5c03b503e2cf202bcbc09fe91aef14
Reviewed-on: https://chromium-review.googlesource.com/804956Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49844}

src/conversions.cc

@@ -953,6 +953,7 @@ MaybeHandle<BigInt> BigIntParseInt(Isolate* isolate, Handle<String> string,
 }
 
 MaybeHandle<BigInt> StringToBigInt(Isolate* isolate, Handle<String> string) {
+  string = String::Flatten(string);
   BigIntParseIntHelper helper(isolate, string);
   return helper.GetResult();
 }

src/runtime/runtime-bigint.cc

@@ -57,7 +57,6 @@ RUNTIME_FUNCTION(Runtime_BigIntEqualToString) {
   DCHECK_EQ(2, args.length());
   CONVERT_ARG_HANDLE_CHECKED(BigInt, lhs, 0);
   CONVERT_ARG_HANDLE_CHECKED(String, rhs, 1);
-  rhs = String::Flatten(rhs);
   bool result = BigInt::EqualToString(lhs, rhs);
   return *isolate->factory()->ToBoolean(result);
 }