My earlier change https://crrev.com/c/1991969 had a bug. If
ARB_texture_rectangle is disabled once, it can't be re-enabled because
the extension behavior entry is deleted. ResetExtensionBehavior needs
to restore it.

Bug: angleproject:3770
Change-Id: Icf96acece8ea8d17287c8d09fb4849b61c9c2a36
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2023398Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: James Darpinian <jdarpinian@chromium.org>