WEBGL_video_texture is an extension that will improve uploading video frame to WebGL performance.
(https://www.khronos.org/registry/webgl/extensions/proposals/WEBGL_video_texture/)

This extension introduced a new texture type TEXTURE_VIDEO_IMAGE_WEBGL and a new sampler type
samplerVideoWEBGL to sample it.

In chromium implementation, TEXTURE_VIDEO_IMAGE_WEBGL maps to different native texture type
based on platform. On desktop, it maps to GL_TEXTURE2D(Currently supported). On Android, it
should map to GL_TEXTURE_EXTERNAL(TODO). SamplerVideoWEBGL needs to be mapped to sampler2D or
samplerExternalOES according to TEXTURE_VIDEO_IMAGE_WEBGL implementation.

This patch implements samplerVideoWEBGL in Angle to support WEBGL_video_texture on desktop. In
this case, samplerVideoWEBGL should map to sampler2D.

Bug: chromium:776222, angleproject:3889
Change-Id: Idb0a5fcde37ca75ccc1181226b91f257212e7500
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1866274
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>