Fix -Wsign-compare warnings in parser, scanner, regexp, runtime.

src/libsampler/sampler.cc

Index: src/libsampler/sampler.cc
diff --git a/src/libsampler/sampler.cc b/src/libsampler/sampler.cc
index 0b40972b8e41d27333124e5e49a66d9713d8943a..f65498aa607b10078cc5ac0a6fb8c929ec6b47fd 100644
--- a/src/libsampler/sampler.cc
+++ b/src/libsampler/sampler.cc
@@ -281,7 +281,7 @@ class SamplerManager {
     if (!entry) return;
     SamplerList& samplers = *static_cast<SamplerList*>(entry->value);
 
-    for (int i = 0; i < samplers.size(); ++i) {
+    for (size_t i = 0; i < samplers.size(); ++i) {
       Sampler* sampler = samplers[i];
       Isolate* isolate = sampler->isolate();
       // We require a fully initialized and entered isolate.