Previously a pixel shader would have gl_ViewID_OVR as input only if it
were used in it. However, in a case with a user-specified varying and no
usage of gl_ViewID_OVR in the pixel shader, a link-time error would
occur whenever the geometry and pixel shaders were to be linked as the
order of parameters in the signatures would not match.
The patch addresses this by always having gl_ViewID_OVR declared as
pixel shader input if the OVR_multiview(2) shader extension is enabled.

The patch does not provide any tests because draw commands are not yet
supported at this stage.

BUG=angleproject:2062
TEST=angle_end2end_tests

Change-Id: Ia154fd53cbf3847cfaf0504e4ec44632b9cd5327
Reviewed-on: https://chromium-review.googlesource.com/641151
Commit-Queue: Martin Radev <mradev@nvidia.com>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>