After unbinding IOSurface textures, we reset the texture size to 0 because
some platforms don't correctly update their state tracking.  This call ends
up generating GL_INVALID_OPERATION errors on Mac for unknown reasons.

Skip the call, the size reset is not required on Mac anyways.

BUG=angleproject:3859

Change-Id: I5e39b6a36aaff41082a5fcc923970f8e97774675
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1769059Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Geoff Lang <geofflang@chromium.org>