Also initialize this structure within Program instead of DynamicHLSL.
This should have benefits for other back-ends. Also these variables
weren't being serialized and de-serialized with the program binary,
which could mess up WebGL apps that use MRT.

BUG=angleproject:1123

Change-Id: Ic0dd4840f26441a1bee8527dfa178b24daf82f8a
Reviewed-on: https://chromium-review.googlesource.com/294571Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>