Also, fix the warnings generated by this flag.

(cherry picked from commit ebeb1155d4fa6d28e2f40c92265245f8df097fcb)

From AOM. Don't actually add -Wsign-compare. It will be covered by
-Wextra.

Switch to vpx_integer.h from df9c9d6d4c43f02c58d4e776c53323788e013cbc

BUG=webm:1069

Change-Id: I1dc6e61caa5d56af4a55b6692ab620bb3144652a