This is an attempt to fix use-of-uninitialized-value errors.
Example failure: https://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_msan_rel_ng/builds/68/steps/cc_unittests%20%28with%20patch%29/logs/EnlargedTextureWithAlphaThresholdFilter.GL

BUG=angleproject:1044

Change-Id: I5906c67a55da553e2e5fd15320a5dd4186644e87
Reviewed-on: https://chromium-review.googlesource.com/287191Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>