Make this a proper class, fix the extends method (previously did not
work as expected), add a contains method, and add tests. Also add an
iterator helper class so we can iterate over the range with range-for
loops.

This also fixes the shader resource unsetting code, which was not
actually unsetting all the possible applied textures.

BUG=angleproject:2052

Change-Id: I2a6fa97f96ccb612ad01a5e3f24dc869c54c967b
Reviewed-on: https://chromium-review.googlesource.com/527318Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>