Similarly to the word32->float64 case, we interpret word32 as uint32 if
the value is word32 truncated. This is fine because the users declared
they only care about mod 2^32 of the value (that's what word32
truncation means).

This CL also removes the ad-hoc handling of this situation
(https://codereview.chromium.org/2311903002).

BUG=chromium:644048

Review-Url: https://codereview.chromium.org/2312003005
Cr-Commit-Position: refs/heads/master@{#39224}