Fix media_cas_proxy_sdk build issue.

Add example binary for testing building the SDK after 'git clone' from our repo.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=227583629

protos/public/signed_drm_certificate.proto

@@ -0,0 +1,27 @@
+////////////////////////////////////////////////////////////////////////////////
+// Copyright 2016 Google LLC.
+//
+// This software is licensed under the terms defined in the Widevine Master
+// License Agreement. For a copy of this agreement, please contact
+// widevine-licensing@google.com.
+////////////////////////////////////////////////////////////////////////////////
+
+// Signed device certificate definition.
+
+syntax = "proto2";
+
+package widevine;
+
+option java_outer_classname = "SignedDrmCertificateProtos";
+option java_package = "com.google.video.widevine.protos";
+
+// DrmCertificate signed by a higher (CA) DRM certificate.
+message SignedDrmCertificate {
+  // Serialized certificate. Required.
+  optional bytes drm_certificate = 1;
+  // Signature of certificate. Signed with root or intermediate
+  // certificate specified below. Required.
+  optional bytes signature = 2;
+  // SignedDrmCertificate used to sign this certificate.
+  optional SignedDrmCertificate signer = 3;
+}