Initialize TextureState::usage. This was probably omittted some time ago, and was showing up as garbage in the debugger in some cases. BUG=angleproject:1260 Change-Id: Ifca5243d0c94e7659a2245e327a72eed9dd918ab Reviewed-on: https://chromium-review.googlesource.com/327401 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>