Fix -Werror=sign-compare error with GCC

src/frames.cc

Index: src/frames.cc
diff --git a/src/frames.cc b/src/frames.cc
index c6b04f7081fc1f9bd6c81cadc7ffc8a8ac11e7b2..cde6f21f74813ddb7f90612293bcc057225929ae 100644
--- a/src/frames.cc
+++ b/src/frames.cc
@@ -960,7 +960,7 @@ void OptimizedFrame::Summarize(List<FrameSummary>* frames) {
           Deoptimizer::GetOutputInfo(output_data, ast_id, shared_info);
       unsigned const pc_offset =
           FullCodeGenerator::PcField::decode(entry) + Code::kHeaderSize;
-      DCHECK_NE(0, pc_offset);
+      DCHECK_NE(0U, pc_offset);
 
       FrameSummary summary(receiver, function, code, pc_offset, is_constructor);
       frames->Add(summary);