Change the default behaviour rtc_builtin_ssl_root_certificates.

Instead of defining a pre-processor macro when someone wants to
include built-in ssl roots certs, this CL switches the default and
assumes everyone prefer to include built-in ssl roots certs.

If built-in ssl roots certs are not needed because they are injected
in the PeerConnection it will be possible to define a pre-processor
macro (WEBRTC_EXCLUDE_BUILT_IN_SSL_ROOT_CERTS) to remove them.

In a GN build it is possible to tell GN to define the macro by setting
rtc_builtin_ssl_root_certificates to false in "gn args".

Bug: webrtc:9332
Change-Id: Icc3f2caeddca6899cbc5974f21b480d75d15556f
Reviewed-on: https://webrtc-review.googlesource.com/94147
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Benjamin Wright <benwright@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24302}
diff --git a/BUILD.gn b/BUILD.gn
index 2e3c91a..b3afa76 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -92,6 +92,10 @@
     defines += [ "WEBRTC_MOZILLA_BUILD" ]
   }
 
+  if (!rtc_builtin_ssl_root_certificates) {
+    defines += [ "WEBRTC_EXCLUDE_BUILT_IN_SSL_ROOT_CERTS" ]
+  }
+
   # Some tests need to declare their own trace event handlers. If this define is
   # not set, the first time TRACE_EVENT_* is called it will store the return
   # value for the current handler in an static variable, so that subsequent