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/266992Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>