Add layout test leak detection trybot config

This CL adds a new trybot to mb_config.

Change-Id: I3947f570e393ac01c01eaf2aef8ef2e6fc7ed2f8
Reviewed-on: https://chromium-review.googlesource.com/885381Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Yuzu Saijo <yuzus@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532327}

tools/mb/mb_config.pyl

@@ -654,6 +654,7 @@
       'linux_vr': 'vr_release_trybot',
       'linux_upload_clang': 'release_bot',
       'leak_detection_linux': 'release_trybot',
+      'layout_test_leak_detection': 'release_trybot',
     },
 
     'tryserver.chromium.mac': {