Fix initialization sequence translation bug.

BUG=angle:805
TEST=https://www.khronos.org/registry/webgl/sdk/tests/conformance/glsl/misc/expression-list-in-declarator-initializer.html

Change-Id: I94fbdd0824dbe73034f8bf022aa53627582ad48b
Reviewed-on: https://chromium-review.googlesource.com/224813
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Zhenyao Mo <zmo@chromium.org>