Add dawn-mac-x86-deps-rel to mb_config.pyl.

Forgotten in the most recent CL.

Bug: 948902
Change-Id: Ic72688b98721fe6081c6902849ae2852c2b7fdc1
Tbr: martiniss@chromium.org
No-Try: True
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1562795Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649757}

tools/mb/mb_config.pyl

@@ -718,6 +718,7 @@
 
     'tryserver.chromium.dawn': {
       'dawn-linux-x64-deps-rel': 'dawn_tests_release_trybot',
+      'dawn-mac-x64-deps-rel': 'dawn_tests_release_trybot',
       'dawn-win10-x86-deps-rel': 'dawn_tests_release_trybot_x86',
       'dawn-win10-x64-deps-rel': 'dawn_tests_release_trybot',
       'linux-dawn-rel': 'gpu_fyi_tests_release_trybot',