Allow only constants, other globals, temporaries, and uniforms to be used
in global variable initialization for now. The spec limits global
variable initialization further to only constant expressions, but fully
implementing this has a too large compatibility cost for ESSL 1.00, so
implement it only partially. In the case of ESSL 3.00 we can use stricter
validation, since there's no legacy to worry about.

Resubmitting the change, since the previous version neglected to remove
incorrect GLSL tests.

TEST=angle_unittests, WebGL conformance tests, angle_end2end_tests
BUG=angleproject:988

Change-Id: I1bb3b8dc305689a90eadfe8cc7705e5ac3829e03
Reviewed-on: https://chromium-review.googlesource.com/270651Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>