Fix media_cas_proxy_sdk build issue.

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

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

example/BUILD

@@ -0,0 +1,39 @@
+################################################################################
+# Copyright 2019 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.
+################################################################################
+
+# Build file for the example code.
+
+package(
+    default_visibility = ["//visibility:public"],
+)
+
+
+filegroup(
+    name = "binary_release_files",
+    srcs = [
+        "wvpl_cas_proxy_environment_example.cc",
+        "wvpl_cas_proxy_session_example.cc",
+        ":wvpl_cas_proxy_environment_example",
+        ":wvpl_cas_proxy_session_example",
+    ],
+)
+
+cc_binary(
+    name = "wvpl_cas_proxy_environment_example",
+    srcs = ["wvpl_cas_proxy_environment_example.cc"],
+    deps = [
+        "//media_cas_proxy_sdk/external/common/wvpl:wvpl_cas_proxy_environment",
+        "//media_cas_proxy_sdk/external/common/wvpl:wvpl_cas_proxy_session",  # build_cleaner: keep
+    ],
+)
+
+cc_binary(
+    name = "wvpl_cas_proxy_session_example",
+    srcs = ["wvpl_cas_proxy_session_example.cc"],
+    deps = ["//media_cas_proxy_sdk/external/common/wvpl:wvpl_cas_proxy_session"],
+)

example/wvpl_cas_proxy_environment_example.cc

@@ -0,0 +1,24 @@
+////////////////////////////////////////////////////////////////////////////////
+// Copyright 2019 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.
+////////////////////////////////////////////////////////////////////////////////
+
+// Example of usage of wvpl_cas_proxy_environment.
+
+#include <iostream>
+#include <map>
+#include <string>
+
+#include "media_cas_proxy_sdk/external/common/wvpl/wvpl_cas_proxy_environment.h"
+
+int main(int argc, char **argv) {
+  std::map<std::string, std::string> config_values;
+  widevine_server::wv_pl_sdk::WvPLCASProxyEnvironment environment(
+      config_values);
+  std::cout << "Hello world!" << std::endl;
+
+  return 0;
+}

example/wvpl_cas_proxy_session_example.cc

@@ -0,0 +1,23 @@
+////////////////////////////////////////////////////////////////////////////////
+// Copyright 2019 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.
+////////////////////////////////////////////////////////////////////////////////
+
+// Example of usage of wvpl_cas_proxy_session.
+
+#include <iostream>
+#include <string>
+
+#include "media_cas_proxy_sdk/external/common/wvpl/wvpl_cas_proxy_session.h"
+
+int main(int argc, char **argv) {
+  std::cout << "Session version: "
+            << widevine_server::wv_pl_sdk::WvPLCASProxySession::
+                   GetVersionString()
+            << std::endl;
+
+  return 0;
+}