In DesktopGL, SRGB blending must be enabled or linear blending will be
used.

Passes all dEQP-GLES3.functional.fragment_ops.blend.fbo_srgb.* tests (1106
new passing tests).

BUG=angleproject:883
BUG=angleproject:885

Change-Id: I0dfb744faa811f10be47c5bd8295b368baf3b04b
Reviewed-on: https://chromium-review.googlesource.com/298620Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>