1) Before this workaround is hardwired on mac, now we move it behind a compil
    2) Fix the issue where "break" inside the loop isn't handled while unrolled.

    BUG=338474
    TEST=webgl conformance test sampler-array-using-loop-index.html

Change-Id: I4996a42c2dea39a8a5af772c256f8e3cb383f59a
Reviewed-on: https://chromium-review.googlesource.com/188079Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Zhenyao Mo <zmo@chromium.org>