Don't try to enable or disable GL_FRAMEBUFFER_SRGB unless
extensions.sRGBWriteControl is true.  For example if you
try it with Mesa GLES2 you get a GL error.

BUG=angleproject:1896

Change-Id: I5f5b4e8ea4f5a7c6913f27761a6e2dc88aacc78c
Reviewed-on: https://chromium-review.googlesource.com/443824
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>