Use an explicit identifier in Config

This let's us use them to configure them when using WebRTC as an external library. One use case where this is necessary is in the Android OS.

Review URL: https://codereview.webrtc.org/1538643004

Cr-Commit-Position: refs/heads/master@{#11231}

webrtc/modules/video_coding/codecs/vp8/temporal_layers.h

@@ -14,6 +14,7 @@
 
 #include "vpx/vpx_encoder.h"
 
+#include "webrtc/common.h"
 #include "webrtc/common_video/include/video_image.h"
 #include "webrtc/typedefs.h"
 
@@ -30,6 +31,8 @@ class TemporalLayers {
     virtual ~Factory() {}
     virtual TemporalLayers* Create(int temporal_layers,
                                    uint8_t initial_tl0_pic_idx) const;
+    static const ConfigOptionID identifier =
+        ConfigOptionID::kTemporalLayersFactory;
   };
 
   virtual ~TemporalLayers() {}