Enable usrsctp runtime checks.

The usrsctp library has a mechanism to enforce invariants (similar to
CHECK or RTC_CHECK). In order to enable these checks, the macro
INVARIANTS needs to be defined on all the usrsctp translation units.

Maybe, this macro should be renamed into USRSCTP_ENABLE_CHECKS or at
least into something with USRSCTP_ as prefix.

This CL enables these runtime checks in debug builds and when the
build is explicitly asking for them (dcheck_always_on=true).

Bug: None
Change-Id: Id63d78200ae2a762b86a4dc0c70d1a04e7d94350
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2600903Reviewed-by: Harald Alvestrand <hta@chromium.org>
Commit-Queue: Mirko Bonadei <mbonadei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840138}

third_party/usrsctp/BUILD.gn

@@ -2,6 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/config/dcheck_always_on.gni")
 import("//build/toolchain/toolchain.gni")
 
 config("usrsctp_config") {
@@ -10,10 +11,18 @@ config("usrsctp_config") {
     "usrsctplib/usrsctplib/netinet",
   ]
 
+  defines = []
+
+  if (is_debug || dcheck_always_on) {
+    defines += [
+      "INVARIANTS",  # Enable runtime checks.
+    ]
+  }
+
   # This is part of the public config because it's used both by usrsctp and
   # the client (WebRTC), to determine if the length field needs populating.
   if (is_mac || is_ios || target_os == "freebsd") {
-    defines = [ "HAVE_SCONN_LEN" ]
+    defines += [ "HAVE_SCONN_LEN" ]
   }
 }