DeallocateCurrent is only called from within DllMain, which means
it's only called on windows.

On the mac, the warning unused-function may be triggered.

Move DeallocateCurrent to windows only code.

Change-Id: I61ba602af78d546a91cf009ae7350cf03eaa84dd
Reviewed-on: https://chromium-review.googlesource.com/305880Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tryjob-Request: Corentin Wallez <cwallez@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>