Previously the code didn't resize the depth buffer. Also adds a magic
setNeedsDisplay call when resizing a CALayer.

BUG=angleproject:1233

Change-Id: Id1906312f58d474e95a1b5ebfc15eff4344cd126
Reviewed-on: https://chromium-review.googlesource.com/318450
Tryjob-Request: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>