Temporarily suppress -Wdeprecated-declarations to update jsoncpp.

This is required to land [1] which introduces new APIs and deprecates
the old ones.

[1] - https://chromium-review.googlesource.com/c/chromium/src/+/1674449

Bug: webrtc:10770
Change-Id: Iceadf1fa1a3324db6c53f5c822c64ed6fc59fc76
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/144021
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28407}

rtc_base/BUILD.gn

@@ -600,7 +600,15 @@ rtc_static_library("rtc_numerics") {
   ]
 }
 
+# TODO(bugs.webrtc.org/10770): Update jsoncpp API usage and remove this.
+config("rtc_json_temporary_suppressions") {
+  if (!is_win || is_clang) {
+    cflags_cc = [ "-Wno-deprecated-declarations" ]
+  }
+}
+
 rtc_source_set("rtc_json") {
+  public_configs = [ ":rtc_json_temporary_suppressions" ]
   defines = []
   sources = [
     "strings/json.cc",