Refactor translating a ESSL shader string into a target language so that
compiler initialization and cleanup code can be reused between test classes.

BUG=angleproject:817
TEST=angle_unittests

Change-Id: Idb229dceb9e17b13ed6ad2a68ab55ed5c968780e
Reviewed-on: https://chromium-review.googlesource.com/275814Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>