Disallow length being called on expressions with side effects ESSL 3.00 definition of expressions does not include calling length on anything other than array names, so enforce this restriction in the parser. TEST=WebGL 2 conformance tests BUG=angleproject:972 Change-Id: I893d3c468ff21cb419b3239738f2a41834298cf2 Reviewed-on: https://chromium-review.googlesource.com/266992 Reviewed-by: Olli Etuaho <oetuaho@nvidia.com> Tested-by: Olli Etuaho <oetuaho@nvidia.com>