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}
diff --git a/webrtc/examples/unityplugin/unity_plugin_apis.h b/webrtc/examples/unityplugin/unity_plugin_apis.h
index ee7d709..529c7d2 100644
--- a/webrtc/examples/unityplugin/unity_plugin_apis.h
+++ b/webrtc/examples/unityplugin/unity_plugin_apis.h
@@ -37,7 +37,11 @@
                                        int number_of_channels,
                                        int number_of_frames);
 
+#if defined(WEBRTC_WIN)
 #define WEBRTC_PLUGIN_API __declspec(dllexport)
+#elif defined(WEBRTC_ANDROID)
+#define WEBRTC_PLUGIN_API __attribute__((visibility("default")))
+#endif
 extern "C" {
 // Create a peerconnection and return a unique peer connection id.
 WEBRTC_PLUGIN_API int CreatePeerConnection(const char** turn_urls,