This optimizes memory and CPU use of symbol table initialization.
This was measured to make a debug build of angle_unittests run around
10% faster on Windows.

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: Icd5272c6cfc0d5122550c403621bfa5944c92a83
Reviewed-on: https://chromium-review.googlesource.com/859958Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>