Although we only used this method in one place that didn't need to
clear depth or stencil, the method itself doesn't seem like it should
be limited. It will become useful in later patches which initialize
depth and stencil textures robustly.

BUG=angleproject:2107

Change-Id: I7dc67608972f2b31c0cb4d309941be10f47d1d47
Reviewed-on: https://chromium-review.googlesource.com/585600Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>