net/url_util: fix std::string::assign invocation: pass string instead of char*.

According to the C++ standard, the three arguments overload
of std::string::assign accepts an std::basic_string:
http://en.cppreference.com/w/cpp/string/basic_string/assign

Fixing the usage, where char* instead of std::string was passed.

This popped up after libc++ became more standard compliant,
and now it throws a compiler error.

BUG=630681

Review-Url: https://codereview.chromium.org/2177863003
Cr-Commit-Position: refs/heads/master@{#407599}

net/base/url_util.cc

@@ -74,8 +74,7 @@ GURL AppendOrReplaceQueryParameter(const GURL& url,
       replaced = true;
       key_value_pair = (param_name + "=" + param_value);
     } else {
-      key_value_pair.assign(input.data(),
-                            key_range.begin,
+      key_value_pair.assign(input, key_range.begin,
                             value_range.end() - key_range.begin);
     }
     if (!output.empty())