So we could generate shaders for Apple using core GL profile.

By switching to core profile, we still pass most WebGL conformance tests 1.0.2 on Linux, but not all, so apparently more work is needed.

However, I think it's OK to check this CL in because this output profile will be only used behind a chromium switch.

BUG=angleproject:933
TEST=webgl conformance tests

Change-Id: Iad70e1aebf82349d3fc5f4116c1d6bc4448193fd
Reviewed-on: https://chromium-review.googlesource.com/255282Tested-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>