This fixes a crash when we try to load a program executable in the
worker thread in the multi-threaded shader compilation. This fixes
the angle_end2end_tests.

BUG=angle:900

Change-Id: Id4494864c7121630c43c2fe64a92674326eabc69
Reviewed-on: https://chromium-review.googlesource.com/245681Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>