This option is used to initialize shared variables to zero at the
beginning of shader execution to avoid compute shaders being able to
read undefined values that could be coming from another webpage or
application.
It's implemented by declaring variables with initial value for HLSL.
For GLSL, it's not allowed to use declaraction initializer for shared
variables, so we need to explicitly assign them to zero at the
beginning of main(). This implementation is only for HLSL.

Bug: chromium:898030

Change-Id: Ic5906500bf4a35cd9a071923f82f32c5e2991be3
Reviewed-on: https://chromium-review.googlesource.com/c/1330310
Commit-Queue: Jie A Chen <jie.a.chen@intel.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>