[build] Add MB configs for devtools trybot

This adds build configs for a devtools release trybot for running blink
layout tests.

TBR=jbudorick@chromium.org

Bug: 1016290
Change-Id: Ieb73cdd6786545a1216165b5cc701c8628182a56
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872092Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708109}

tools/mb/mb_config.pyl

@@ -924,6 +924,11 @@
       'win_upload_clang': 'release_bot',
     },
 
+    'tryserver.devtools-frontend': {
+      # Use pure release as CI doesn't have a debug builder.
+      'devtools_frontend_linux_blink_rel': 'release_bot',
+    },
+
     'tryserver.v8': {
       'v8_linux_blink_rel': 'release_trybot',
       'v8_linux_chromium_gn_rel': 'release_trybot',