Empty struct declarations with qualifiers are rejected by NVIDIA GL
driver version 367.27. For example this kind of construct that is
expected to be accepted by the WebGL conformance tests is rejected:

const struct a {
    int i;
};

Since qualifiers do not carry meaning unless a struct declaration
has declarators, they can be simply omitted from the translator
output in this kind of cases to work around this driver issue.

New unit test is added to check that pruning empty declarations works
correctly.

BUG=angleproject:1430
BUG=622492
TEST=WebGL conformance tests, angle_unittests

Change-Id: Id83f83124ae597fcdfa15100d336c2c207d9449c
Reviewed-on: https://chromium-review.googlesource.com/356362Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>