mb_config.pyl: add linux-dawn-rel and mac-dawn-rel

BUG=chromium:901830

Change-Id: I8cea07cc08ca559014cf68d75f4eef65a838efb0
Reviewed-on: https://chromium-review.googlesource.com/c/1341844Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609635}

tools/mb/mb_config.pyl

@@ -621,6 +621,8 @@
     },
 
     'tryserver.chromium.dawn': {
+      'linux-dawn-rel': 'gpu_fyi_tests_release_trybot',
+      'mac-dawn-rel': 'gpu_fyi_tests_release_trybot',
       'win-dawn-rel': 'gpu_fyi_tests_release_trybot_x86',
     },