Do this by simply disallowing all function calls in global initializers
- all built-in math ops are handled as other ops, not function calls.

This change was tested extensively with popular WebGL content, with no
regressions found.

TEST=angle_unittests
BUG=angleproject:988

Change-Id: Id1107fa294ae4012d5dd3949539d0b7b4cd21943
Reviewed-on: https://chromium-review.googlesource.com/283703Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>