Suppress warning of use of deprecated OSAtomic* API in usrsctp

Chromoting is planning to bump up iOS deployment target to 10.0. However,
one of our dependency, //third_party/usrsctp has used some OSAtomic*
functions that have been deprecated on 10.0 SDK.

After some discussions with the usrsctp people, we would like to suppress
the deprecation warning on usrsctp for now because:
* Replacing the OSAtomic* functions with atomic_fetch_add is nontrivial
  and requires replacing some int32_t with _Atomic(int32_t) globally in
  their codebase, which is challenging for an engineer who isn't familiar
  with it.
* The OSAtomic* API works all the way up to deployment target 11, if
  ignoring the warnings.
* Filed a bug in their repo but they don't plan to fix that soon.
* They have already suppressed the warning in their configure.ac file.
  This is replicating their change.

Bug: 774483
Change-Id: Ie7f65edb767cc241a8b5fe7dc6074ef37387d89f
Reviewed-on: https://chromium-review.googlesource.com/726619Reviewed-by: Rohit Rao (ping after 24h) <rohitrao@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Reviewed-by: Taylor Brandstetter <deadbeef@chromium.org>
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509905}

third_party/usrsctp/BUILD.gn

@@ -13,8 +13,15 @@ config("usrsctp_config") {
 
 config("usrsctp_warnings") {
   if (is_clang) {
+    cflags = [
       # atomic_init in user_atomic.h is a static function in a header.
-    cflags = [ "-Wno-unused-function" ]
+      "-Wno-unused-function",
+
+      # Use of deprecated OSAtomic* functions in user_atomic.h. This is
+      # what they did in configure.ac. We can remove this once
+      # https://github.com/sctplab/usrsctp/issues/177 is fixed.
+      "-Wno-deprecated-declarations",
+    ]
   }
 }