Enable software decoding video with libvpx on Android

VpxVideoDecoder previously would reject VP8 without an alpha
channel, leaving it to be decoded by FFmpegVideoDecoder. Now,
when it's compiled without FFmpegVideoDecoder it will handle
all VP8.

BUG=507834

Review URL: https://codereview.chromium.org/1281433002

Cr-Commit-Position: refs/heads/master@{#342405}

media/filters/vpx_video_decoder.cc

@@ -294,10 +294,12 @@ bool VpxVideoDecoder::ConfigureDecoder(const VideoDecoderConfig& config) {
   if (config.codec() != kCodecVP8 && config.codec() != kCodecVP9)
     return false;
 
-  // In VP8 videos, only those with alpha are handled by VpxVideoDecoder. All
-  // other VP8 videos go to FFmpegVideoDecoder.
+#if !defined(DISABLE_FFMPEG_VIDEO_DECODERS)
+  // When FFmpegVideoDecoder is available it handles VP8 that doesn't have
+  // alpha, and VpxVideoDecoder will handle VP8 with alpha.
   if (config.codec() == kCodecVP8 && config.format() != PIXEL_FORMAT_YV12A)
     return false;
+#endif
 
   CloseDecoder();