angle: move DeallocateCurrent into windows only code
angle: move DeallocateCurrent into windows only code

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/305880
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Corentin Wallez <470c533e86f4f848c98c7e11fffc2d043494a401@chromium.org>
Tryjob-Request: Corentin Wallez <470c533e86f4f848c98c7e11fffc2d043494a401@chromium.org>
Tested-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

ppy/angle,ecoal95/angle,domokit/waterfall,mlfarrell/angle,MSOpenTech/angle,mikolalysenko/angle,MSOpenTech/angle,ecoal95/angle,ecoal95/angle,ecoal95/angle,ppy/angle,mikolalysenko/angle,ecoal95/angle,mlfarrell/angle,ppy/angle,mlfarrell/angle,mlfarrell/angle,MSOpenTech/angle,domokit/waterfall,mikolalysenko/angle,ppy/angle,mikolalysenko/angle,MSOpenTech/angle