Added casts to remove warnings:
BUG=webm:1274

In regards to the safety of these casts they are of two types:-

- Normalized bits per (16x16) MB stored in a 32 bit int (This is safe as bits
per MB even with << 9 normalization cant overflow 32 bits. Even raw 12
bits hdr source even would only be  29 bits :- (4+4+12+9) and the encoder
imposes much stricter limits than this on max bit rate.

- Cast as part of variance calculations.  There is an internal cast up to 64 bit
for the Sum X Sum calculation, but after normalization dividing by the number
of points the result will always be <= the SSE value.

Change-Id: I4e700236ed83d6b2b1955e92e84c3b1978b9eaa0