Clean up ensureRenderTarget(). We can use the same code for this method, with the only difference being in when we determine a texture image holds no pixels. BUG=angle:729 Change-Id: I3356f5deb900c24b1a5c739ce8de99e9ce1371c0 Reviewed-on: https://chromium-review.googlesource.com/219861 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>