Fix few trivial Coverity issues, mainly PASS_BY_VALUE.

net/base/x509_certificate.h

Index: net/base/x509_certificate.h
diff --git a/net/base/x509_certificate.h b/net/base/x509_certificate.h
index 5956826aef572cd0b83bf910b56c7480e6587c74..1e2419e24e72ad4926a0be21c7244f2daf22e8b2 100644
--- a/net/base/x509_certificate.h
+++ b/net/base/x509_certificate.h
@@ -75,7 +75,7 @@ class X509Certificate : public base::RefCountedThreadSafe<X509Certificate> {
   // Principal represent an X.509 principal.
   struct Principal {
     Principal() { }
-    explicit Principal(std::string name) : common_name(name) { }
+    explicit Principal(const std::string& name) : common_name(name) { }
 
     // The different attributes for a principal.  They may be "".
     // Note that some of them can have several values.