RemoveSwitchFallThrough now treats cases where break; or return; is
nested inside a block as non-fallthrough to avoid unnecessary
duplication of code. For example, the case 1 below would previously
get treated as fall-through:

switch(foo)
{
    case 1:
    {
        break;
    }
    default:
        break;
}

Now RemoveSwitchFallThrough doesn't do anything to this code.

BUG=chromium:772695
TEST=angle_end2end_tests

Change-Id: Iafab6d8b05c63bcdb5f54834dbc1f41192c31dd4
Reviewed-on: https://chromium-review.googlesource.com/709197Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>