This patch fixes a compile error when we do iteration on ShaderBitSet.
Now we can directly get a ShaderType variable in a range-for iteration
on a ShaderBitSet.

BUG=angleproject:2169

Change-Id: I23e38f2ebd1c72145a2e54be374f7dcd9f5fb9e2
Reviewed-on: https://chromium-review.googlesource.com/1100312
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jiajia Qin <jiajia.qin@intel.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>