Add base/parse_number.h to generalize number parsing done in //net.

net/base/host_port_pair.cc

Index: net/base/host_port_pair.cc
diff --git a/net/base/host_port_pair.cc b/net/base/host_port_pair.cc
index a0dd2d507520df724bad704b35e81fdc11360d4b..441a35965d8f4fa7e93a8b323d1a4ffca19d1d20 100644
--- a/net/base/host_port_pair.cc
+++ b/net/base/host_port_pair.cc
@@ -10,6 +10,7 @@
 #include "base/strings/string_util.h"
 #include "base/strings/stringprintf.h"
 #include "net/base/ip_endpoint.h"
+#include "net/base/parse_number.h"
 #include "net/base/port_util.h"
 #include "url/gurl.h"
 
@@ -37,7 +38,7 @@ HostPortPair HostPortPair::FromString(const std::string& str) {
   if (key_port.size() != 2)
     return HostPortPair();
   int port;
-  if (!base::StringToInt(key_port[1], &port))
+  if (!ParseNonNegativeDecimalInt(key_port[1], &port))
     return HostPortPair();
   if (!IsPortValid(port))
     return HostPortPair();