[CFI] Remove function pointer casts
[CFI] Remove function pointer casts

Control Flow Integrity [1] indirect call checking verifies that function
pointers only call valid functions with a matching type signature. This
change eliminates some function pointer casts that I missed in my last
CL https://crrev.com/c/780144.

BUG=chromium:776905

[1] https://www.chromium.org/developers/testing/control-flow-integrity

Change-Id: I1c7adbdfffa4fe0b62e993bfb31d06e64b022d66

webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx