The ESSL 1.0 spec is clear that "initializers must be a constant expression."
Yet, because of "legacy" applications (apparently just WebGL applications), the
code was only issuing a warning and not an error.  The
"KHR-GLES2.shaders.negative.initialize" test requires an error be generated.
This change splits the semantics, allowing GLES applications to get an error,
and WebGL applications to get a warning.

Note: This change is related to https://chromium-review.googlesource.com/829138
(for angleproject:2285).

Bug: angleproject:3381
Change-Id: Ie243b7dd72102aeb52df506d121d1d2a8f6974d3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1716617Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Ian Elliott <ianelliott@google.com>