The GL backend was checking the wrong extension for GL_ETC1_RGB8_OES.

WebGL doesn't want to expose compressed formats that are emulated,
disable this format in D3D11 where it is always emulated.

BUG=98314

Change-Id: I1ba8418ad1578c070891828b87e49cf894dabc29
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1700574Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>