This allows the regular BindingPointer class to template on const types.

Also added assignment and copy operators that don't leak the resources.

BUG=angleproject:880

Change-Id: If3efa26a7fa89306d783c3e8a5fb92b16861d77a
Reviewed-on: https://chromium-review.googlesource.com/257670Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>