Remove libvpx from FFmpeg in favour of ffvp8.

ffmpeg/source/avcodec-53.novpx_sigs

-# Copyright (c) 2010 The Chromium Authors. All rights reserved.
-# Use of this source code is governed by a BSD-style license that can be
-# found in the LICENSE file.
-#
-# Functions from avcodec used in chromium code.
-
-AVCodecContext *avcodec_alloc_context(void);
-AVCodec *avcodec_find_decoder(enum CodecID id);
-AVFrame *avcodec_alloc_frame(void);
-int av_get_bits_per_sample_fmt(enum SampleFormat sample_fmt);
-int av_new_packet(AVPacket *pkt, int size);
-int avcodec_decode_audio3(AVCodecContext *avctx, int16_t *samples, int *frame_size_ptr, AVPacket *avpkt);
-int avcodec_decode_video2(AVCodecContext *avctx, AVFrame *picture, int *got_picture_ptr, AVPacket *avpkt);
-int avcodec_open(AVCodecContext *avctx, AVCodec *codec);
-int avcodec_close(AVCodecContext *avctx);
-int avcodec_thread_init(AVCodecContext *s, int thread_count);
-void av_free_packet(AVPacket *pkt);
-void av_init_packet(AVPacket *pkt);
-int av_dup_packet(AVPacket *pkt);
-void avcodec_flush_buffers(AVCodecContext *avctx);
-void avcodec_init(void);
-AVBitStreamFilterContext *av_bitstream_filter_init(const char *name);
-int av_bitstream_filter_filter(AVBitStreamFilterContext *bsfc, AVCodecContext *avctx, const char *args, uint8_t **poutbuf, int *poutbuf_size, const uint8_t *buf, int buf_size, int keyframe);
-void av_bitstream_filter_close(AVBitStreamFilterContext *bsf);
-void av_destruct_packet(AVPacket *pkt);
-int av_lockmgr_register(AVLockMgrOperation cb);
-
-# RDFT functions.
-RDFTContext *av_rdft_init(int nbits, enum RDFTransformType trans);
-void av_rdft_calc(RDFTContext *s, FFTSample *data);
-void av_rdft_end(RDFTContext *s);