glEnable/glDisable GL_FRAMEBUFFER_SRGB isn't meaningful on GLES.

BUG=angleproject:1300

Change-Id: I0afd697ed9770e2eb1cdcc851884b90de1a4583c
Reviewed-on: https://chromium-review.googlesource.com/334490Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>