Because this builtin uses a structure, certain shaders could trigger
the mangled name to be allocated during normal shader compilation.
Then when the scope is popped, the mangled name for DepthRange is
freed, and we're left with a dangling pointer.

Fix this temporarily by enforcing mangled name construction when
we initialize the builtins, but we should look for a more robust and
future-proof fix.

BUG=620937

Change-Id: If130c8b48a18054502abaec08f10264f282b4925
Reviewed-on: https://chromium-review.googlesource.com/354494Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Antoine Labour <piman@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/360480Reviewed-by: Jamie Madill <jmadill@chromium.org>