Build the ANGLE dEQP tests on linux_optional_gpu_tests_rel

Tryjobs on this bot try to run the angle_deqp_gles3_tests but it
isn't found causing gn gen to fail. This fixes this issue by adding
the correct gn flag to make ANGLE generate the dEQP targets.

BUG=583702

Review URL: https://codereview.chromium.org/1809803003

Cr-Commit-Position: refs/heads/master@{#381580}

tools/mb/mb_config.pyl

@@ -330,6 +330,7 @@
 
     'swarming_gpu_fyi_tests_gn_release_trybot': [
       'swarming', 'gpu_tests', 'gn', 'release_trybot',
+      'angle_deqp_tests',
     ],
 
     'swarming_gpu_tests_gn_debug_bot': [