The fact that passing shaders to the platform compiler is deferred until
linking is performed allowed this bug to pass undetected in earlier manual
testing.

TEST=WebGL conformance tests
BUG=angleproject:941

Change-Id: Ic12b5d0c5547057551df90086453e897a161d559
Reviewed-on: https://chromium-review.googlesource.com/263221Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Nicolas Capens <capn@chromium.org>