Emulation of this extension would potentially require rewriting ESSL
1.00 shaders as 3.00 when the extension is missing, and we're on a
GLES 3.0 context. For now, it's easier to disable the extension when
native support is lacking.

BUG=angleproject:1828

Change-Id: I8eabb8efccc2ddd5fafd3521657ea68cc9e1d1e0
Reviewed-on: https://chromium-review.googlesource.com/436144
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>