Move Framebuffer clearing from Renderer to the Framebuffer object. BUG=angle:841 Change-Id: I95c9cbdc2d1c99731e19c48e18117358d22b9e94 Reviewed-on: https://chromium-review.googlesource.com/232690 Reviewed-by: Kenneth Russell <kbr@chromium.org> Reviewed-by: Brandon Jones <bajones@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>