Cherry pick buffer re-allocation fix

https://chromium-review.googlesource.com/#/c/272379/

[decoder] Optimize context buffer re-allocation

1. Check existing buffer sizes when re-allocate context buffers.
2. Don't need to set mi buffers to 0 during setup_mi.

BUG=495245

Review URL: https://codereview.chromium.org/1155073005
3 files changed