media/gpu/test: Fix crash when running video decoder allocate mode test.

This CL fixes a crash in the video_decode_accelerator_tests when running the
FlushAtEndOfStream_Allocate test that was added in crrev.com/c/1688750.

Using the frame validator is not supported when using allocate mode, but the
check to enable the frame validator was incorrect.

TEST=./video_decode_accelerator_tests on eve

BUG=None

Change-Id: I94ad6cfc74bf93c58ca0cfe215df3a31f093c125
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1699385
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Commit-Queue: David Staessens <dstaessens@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676727}