This frees the Framebuffer from any dependency on the Renderer.

BUG=angle:789

Change-Id: I0e9ae0edf93674e32ae4f6538a934cddbdc804d5
Reviewed-on: https://chromium-review.googlesource.com/225470Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>