Initializing globals is done by the DeferGlobalInitializers AST
transformation, which makes sure that the global variable init
proceeds in the correct order. This is important since some global
variables may use other uninitialized globals in their initializers.

The global variable init is only done in case the WebGL shader spec is
being followed. This avoids adding global initializers twice in case
the same shader goes through multiple translations, assuming only one
of them is a WebGL shader translation.

BUG=angleproject:1966
TEST=angle_end2end_tests, WebGL conformance tests

Change-Id: Iea701d3ae40edc906abd87303f2aa27fd23bf55a
Reviewed-on: https://chromium-review.googlesource.com/509689
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>