Fix double delete with invariant varyings.

The compiler would leave some TString variables lying around
after the pool gets released, leading to a potential crash.

BUG=angle:846

Change-Id: I484ed9b14bba9bf653f6ed4001ae79f87791b0dd
Reviewed-on: https://chromium-review.googlesource.com/232780
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>