Expose GL_CHROMIUM_compressed_texture_etc on top of ES drivers. This allows Chrome to expose the ETC texture formats in WebGL on top of ANGLE. BUG=angleproject:1552 Change-Id: I6a6c3912791e121826b2083421e37df7c0dc38e7 Reviewed-on: https://chromium-review.googlesource.com/c/1342420 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Kai Ninomiya <kainino@chromium.org>