Call ShDestruct() to destroy the compiler objects to
avoid memory leaks.

BUG=angle:737

Change-Id: I71a8ddfe67c9d8c7b4e5b5683c69dd578fc38c66
Reviewed-on: https://chromium-review.googlesource.com/215860Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Sudarsana Nagineni <sudarsana.nagineni@intel.com>