Fix ParseCIDRBlock() to no longer accept invalid inputs.

net/base/ip_address.cc

Index: net/base/ip_address.cc
diff --git a/net/base/ip_address.cc b/net/base/ip_address.cc
index 42071e010e358be1b30e46431574de9e5dd116f2..7ae68bf44a951587c5776d966945419306595ffa 100644
--- a/net/base/ip_address.cc
+++ b/net/base/ip_address.cc
@@ -158,9 +158,15 @@ bool ParseCIDRBlock(const std::string& cidr_literal,
   if (!ip_address->AssignFromIPLiteral(parts[0]))
     return false;
 
+  // TODO(martijnc): Find a more general solution for the overly permissive
+  // base::StringToInt() parsing. https://crbug.com/596523.
+  const base::StringPiece& prefix_length = parts[1];
+  if (prefix_length.starts_with("+"))
+    return false;
+
   // Parse the prefix length.
   int number_of_bits = -1;
-  if (!base::StringToInt(parts[1], &number_of_bits))
+  if (!base::StringToInt(prefix_length, &number_of_bits))
     return false;
 
   // Make sure the prefix length is in a valid range.