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/drm_certificate.proto

@@ -27,7 +27,9 @@ message DrmCertificate {
     SERVICE = 3;
     PROVISIONER = 4;
   }
-
+  enum ServiceType {
+    UNKNOWN = 0; LICENSE_SERVER_SDK = 1; LICENSE_SERVER_PROXY_SDK = 2;
+  }
   // Type of certificate. Required.
   optional Type type = 1;
   // 128-bit globally unique serial number of certificate.
@@ -47,4 +49,7 @@ message DrmCertificate {
   // Service identifier (web origin) for the provider which owns the
   // certificate. Required for service and provisioner certificates.
   optional string provider_id = 7;
+  // This field is used only when type = SERVICE to specify which SDK uses
+  // service certificate.
+  optional ServiceType service_type = 8 [default = UNKNOWN];
 }