The patch adds the specification for the ANGLE_texture_multisample
extension, which enables the functionality of multisample texture.

This extension is also the basis to expose multisample texture to
web via a webgl extension.

BUG=angleproject:2275
BUG=angleproject:2775

Change-Id: I21771ac1d7d3e2fad8bf5d5e0a5c5e082c8fe7a2
Reviewed-on: https://chromium-review.googlesource.com/859917
Commit-Queue: Yunchao He <yunchao.he@intel.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>