[webrtc] Add missing header in client code (continued).

We were relying on brittle transitive #include.

Bug: webrtc:8311
Change-Id: I1749d9159c4fbff6658f19fefa84f46b0acabf5c
Reviewed-on: https://chromium-review.googlesource.com/c/1290935
Commit-Queue: Yves Gerey <yvesg@google.com>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601517}

third_party/blink/renderer/modules/peerconnection/rtc_certificate.cc

@@ -33,6 +33,7 @@
 #include "base/memory/ptr_util.h"
 #include "third_party/blink/renderer/platform/bindings/to_v8.h"
 #include "third_party/blink/renderer/platform/bindings/v8_binding.h"
+#include "third_party/webrtc/rtc_base/sslcertificate.h"
 
 namespace blink {