The access in fillCurrentRequestPostBody wasn't protected with
synchronization. Furthermore, just synchronizing it wouldn't be
sufficient, since what we really need to check is whether the
Content-Type header has been added to the UrlRequest.Builder.
The contents of requestProperties may have changed between the
headers being added to UrlRequest.Builder and the call to
fillCurrentRequestPostBody.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=135376904
