Fix build with Heimdal.

net/http/http_auth_gssapi_posix.cc

Index: net/http/http_auth_gssapi_posix.cc
diff --git a/net/http/http_auth_gssapi_posix.cc b/net/http/http_auth_gssapi_posix.cc
index 431cfc4f2b54730fa6b46a6801c25cf691b7c878..aff2ac1114e09ec202b1ce4bd8d63ebd41483632 100644
--- a/net/http/http_auth_gssapi_posix.cc
+++ b/net/http/http_auth_gssapi_posix.cc
@@ -55,6 +55,16 @@ static gss_OID_desc GSS_C_NT_EXPORT_NAME_VAL = {
 
 }  // namespace
 
+// Heimdal >= 1.4 will define the following as preprocessor macros.
+// To avoid conflicting declarations, we have to undefine these.
+#undef GSS_C_NT_USER_NAME
+#undef GSS_C_NT_MACHINE_UID_NAME
+#undef GSS_C_NT_STRING_UID_NAME
+#undef GSS_C_NT_HOSTBASED_SERVICE_X
+#undef GSS_C_NT_HOSTBASED_SERVICE
+#undef GSS_C_NT_ANONYMOUS
+#undef GSS_C_NT_EXPORT_NAME
+
 gss_OID GSS_C_NT_USER_NAME = &GSS_C_NT_USER_NAME_VAL;
 gss_OID GSS_C_NT_MACHINE_UID_NAME = &GSS_C_NT_MACHINE_UID_NAME_VAL;
 gss_OID GSS_C_NT_STRING_UID_NAME = &GSS_C_NT_STRING_UID_NAME_VAL;