Add Android Camera To Unity Plugin

The existing unity plugin (an example in webrtc codebase) does not support camera access on Android platform. This CL implements such functionality.

TBR=gyzhou@chromium.org

BUG=webrtc:8067

Review-Url: https://codereview.webrtc.org/2993273002
Cr-Commit-Position: refs/heads/master@{#19277}

webrtc/examples/unityplugin/classreferenceholder.h

@@ -0,0 +1,37 @@
+/*
+ *  Copyright 2015 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+// This is a supplement of webrtc_jni::ClassReferenceHolder.
+// The purpose of this ClassReferenceHolder is to load the example
+// specific java class into JNI c++ side, so that our c++ code can
+// call those java functions.
+
+#ifndef WEBRTC_EXAMPLES_UNITYPLUGIN_CLASSREFERENCEHOLDER_H_
+#define WEBRTC_EXAMPLES_UNITYPLUGIN_CLASSREFERENCEHOLDER_H_
+
+#include <jni.h>
+#include <map>
+#include <string>
+#include <vector>
+
+namespace unity_plugin {
+
+// LoadGlobalClassReferenceHolder must be called in JNI_OnLoad.
+void LoadGlobalClassReferenceHolder();
+// FreeGlobalClassReferenceHolder must be called in JNI_UnLoad.
+void FreeGlobalClassReferenceHolder();
+
+// Returns a global reference guaranteed to be valid for the lifetime of the
+// process.
+jclass FindClass(JNIEnv* jni, const char* name);
+
+}  // namespace unity_plugin
+
+#endif  // WEBRTC_EXAMPLES_UNITYPLUGIN_CLASSREFERENCEHOLDER_H_