Use input framerate as timebase if none was explicitly specified.

Use input framerate as timebase if none was explicitly specified.

After patch:
$ vpxenc tennis_sif.y4m --end-usage=q -q 20 -o x.ivf --ivf --codec=vp9 --limit=30
$ ffmpeg -i x.ivf -i tennis_sif.y4m -lavfi psnr -f null -
[..]
[Parsed_psnr_0 @ 0x7f8913c21e40] PSNR y:43.270594 u:46.300362 v:47.011172 average:44.097994 min:43.460704 max:52.509271

Before patch:
$ vpxenc tennis_sif.y4m --end-usage=q -q 20 -o x.ivf --ivf --codec=vp9 --limit=30
$ ffmpeg -i x.ivf -i tennis_sif.y4m -lavfi psnr -f null -
[..]
[Parsed_psnr_0 @ 0x7fbdaac22780] PSNR y:27.427366 u:41.505376 v:36.853477 average:28.990890 min:22.219454 max:52.509271
$ ffmpeg -i x.ivf -f rawvideo x.yuv
$ ffmpeg -f rawvideo -vcodec rawvideo -video_size 352x240 -framerate 29.97 -i x.yuv -i tennis_sif.y4m -lavfi psnr -f null -
[..]
[Parsed_psnr_0 @ 0x7fadf9e013a0] PSNR y:43.270594 u:46.300362 v:47.011172 average:44.097994 min:43.460704 max:52.509271

Change-Id: Ifac2d6c2ff8379b2d318fe5cfe4be0bf1bc9f27f