1) Do not use gflags in the example binary code.

2) Also surface wv_cas_key_fetcher_example to partner.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=229965302

example/wv_cas_key_fetcher_example.cc

@@ -0,0 +1,68 @@
+////////////////////////////////////////////////////////////////////////////////
+// Copyright 2018 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.
+////////////////////////////////////////////////////////////////////////////////
+
+
+#include <iostream>
+#include <string>
+
+#include "gflags/gflags.h"
+#include "glog/logging.h"
+#include "common/status.h"
+#include "media_cas_packager_sdk/public/wv_cas_key_fetcher.h"
+#include "protos/public/media_cas_encryption.pb.h"
+
+DEFINE_string(content_id, "21140844", "Content ID");
+DEFINE_bool(key_rotation, true, "Whether key rotation is enabled");
+DEFINE_string(track_type, "SD", "Provider name");
+
+int main(int argc, char **argv) {
+  gflags::ParseCommandLineFlags(&argc, &argv, true);
+  CHECK(!FLAGS_content_id.empty() && !FLAGS_track_type.empty())
+      << "Flags 'content_id' and 'track_type' are required";
+  // Required flags in key fetcher.
+  CHECK(!FLAGS_license_server.empty() && !FLAGS_signing_provider.empty() &&
+        !FLAGS_signing_key.empty() && !FLAGS_signing_iv.empty())
+      << "Flags 'license_server', 'signing_provider', 'signing_key' "
+         "and 'signing_iv' are required";
+
+  std::string request_str;
+  widevine::CasEncryptionRequest request;
+  request.set_provider(FLAGS_signing_provider);
+  request.set_content_id(FLAGS_content_id);
+  request.set_key_rotation(FLAGS_key_rotation);
+  // Only 1 track in this example.
+  request.add_track_types(FLAGS_track_type);
+  LOG(INFO) << "Request: " << request.ShortDebugString();
+  if (!request.SerializeToString(&request_str)) {
+    LOG(ERROR) << "Failed to serialize request";
+    return -1;
+  }
+
+  std::string signed_response_str;
+  widevine::cas::WvCasKeyFetcher key_fetcher;
+  widevine::Status status =
+      key_fetcher.RequestEntitlementKey(request_str, &signed_response_str);
+  if (!status.ok()) {
+    LOG(ERROR) << "Failed to request entitlement key";
+    return -1;
+  }
+  widevine::SignedCasEncryptionResponse signed_response;
+  if (!signed_response.ParseFromString(signed_response_str)) {
+    LOG(ERROR) << "Failed to deserialize signed response";
+    return -1;
+  }
+  LOG(INFO) << "Signed response: " << signed_response.ShortDebugString();
+  widevine::CasEncryptionResponse response;
+  if (!response.ParseFromString(signed_response.response())) {
+    LOG(ERROR) << "Failed to deserialize response";
+    return -1;
+  }
+  LOG(INFO) << "Response: " << response.ShortDebugString();
+
+  return 0;
+}