[bigint] Implement BigInt::LessThan

Bug: v8:6791
Change-Id: Ib807ac94dca646cddfe2eb5fafe18999cf0764a2
Reviewed-on: https://chromium-review.googlesource.com/714450Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48508}

src/objects/bigint.cc

@@ -166,7 +166,11 @@ MaybeHandle<BigInt> BigInt::UnsignedRightShift(Handle<BigInt> x,
 }
 
 bool BigInt::LessThan(Handle<BigInt> x, Handle<BigInt> y) {
-  UNIMPLEMENTED();  // TODO(jkummerow): Implement.
+  if (x->sign() == y->sign()) {
+    int result = AbsoluteCompare(x, y);
+    return x->sign() ? (result > 0) : (result < 0);
+  }
+  return x->sign();
 }
 
 bool BigInt::Equal(BigInt* x, BigInt* y) {