Fix few trivial Coverity issues, mainly PASS_BY_VALUE.

base/simple_thread.h

Index: base/simple_thread.h
diff --git a/base/simple_thread.h b/base/simple_thread.h
index 17a2f6d1d3c4f7b1ef06124bc3152ab1c472bd9d..737b97d97f104a787cbb4f46f8e046a5e2e26b2e 100644
--- a/base/simple_thread.h
+++ b/base/simple_thread.h
@@ -152,7 +152,7 @@ class DelegateSimpleThreadPool : public DelegateSimpleThread::Delegate {
  public:
   typedef DelegateSimpleThread::Delegate Delegate;
 
-  DelegateSimpleThreadPool(const std::string name_prefix, int num_threads)
+  DelegateSimpleThreadPool(const std::string& name_prefix, int num_threads)
       : name_prefix_(name_prefix), num_threads_(num_threads),
         dry_(true, false) { }
   ~DelegateSimpleThreadPool();