Remove __stdcall from non-windows builds.

Change-Id: I424cb538808ec64137c85af3bbe8742043e1c573
Reviewed-on: https://chromium-review.googlesource.com/183260
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>