The SRGB blit workaround had to wrong assumptions:
 - SRGB blits can have a multisample source.
 - The woarkound is needed even when the filter is GL_LINEAR in the
   case where we are doing a RGB -> SRGB or RGB -> SRGB blit.

BUG=angleproject:1492
BUG=chromium:658898

Change-Id: I1d89572565a4e23c1c97bdf985bb21a445e898b7
Reviewed-on: https://chromium-review.googlesource.com/409540Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>