Revised coding using adaptive mode context to depend on frame type

A previous commit 76feb965 made the vp8_mode_context adaptive on a frame
frame basis, this commit further made the coding context adaptive to two
frame types separately. Tests on derf set showed a further small gain on
all metrics: avg psnr 0.10%, glb psnr: 0.11%, ssim: 0.08%

http://www.corp.google.com/~yaowu/no_crawl/newNearMode_1209.html

Change-Id: I7b3e32ec8729de1903d14a3f1213f1624b78cdee