Added VRLayer, VRWebGLLayer, and VRViewport

WebGL Layer doesn't yet create a framebuffer, because that's way too
much additional code to cram into this CL.

Bug: 670510
Change-Id: I5d64d264d94f76ddca9eb3add5a651a3de5b2ccc
Reviewed-on: https://chromium-review.googlesource.com/752305
Commit-Queue: Brandon Jones <bajones@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513880}