Disable returning BGRA as an implementation read format unless
GL_EXT_texture_format_BGRA8888 is exposed.  WebGL doesn't support this extension
and returning BGRA as an implementation read format is invalid.

TEST=functional/gles3/negativebufferapi.html
TEST=functional/gles3/integerstatequery.html

BUG=angleproject:4179

Change-Id: I313721f09da0e455128b88b129bd34338d47af73
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1943407Reviewed-by: James Darpinian <jdarpinian@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Geoff Lang <geofflang@chromium.org>