NumNamedElements() should take an enum param.

- Move TAG_ID to webp/mux.h
- Rename it to WebPChunkId
- Rename IDs to WEBP_CHUNK_<tag>
- Remove "name" param from ChunkInfo struct and related changes.
- Rename WebPMuxNumNamedElements to WebPMuxNumChunks().
- WebPMuxNumChunks() takes WebPChunkId as param.

Change-Id: Ic6546e4a9ab823b556cdbc600faa137076546a2b

examples/vwebp.c

@@ -304,7 +304,7 @@ int main(int argc, char *argv[]) {
     if (mux_err != WEBP_MUX_OK && mux_err != WEBP_MUX_NOT_FOUND) {
       goto Error;
     }
-    mux_err = WebPMuxNumNamedElements(kParams.mux, "image",
+    mux_err = WebPMuxNumChunks(kParams.mux, WEBP_CHUNK_IMAGE,
                                       &kParams.frame_max);
     if (mux_err != WEBP_MUX_OK) {
       goto Error;