Add unit test for vp9_ext_ratectrl

Fix three bugs along the way.
1) Call vp9_extrc_send_firstpass_stats() after vp9_extrc_create()
2) Pass in model pointer in vp9_extrc_create()
3) Free frame_stats buffer in vp9_extrc_delete()

Bug: webm:1707

Change-Id: Ic8bd62c7b4ebd85a7479ae5e4c82d7f6059d782f
7 files changed