Add linux dbg coverage to CQ.

NOTRY=true

Review URL: https://codereview.chromium.org/953913002

Cr-Commit-Position: refs/heads/master@{#26830}

PRESUBMIT.py

@@ -244,6 +244,7 @@ def GetPreferredTryMasters(project, change):
   return {
     'tryserver.v8': {
       'v8_linux_rel': set(['defaulttests']),
+      'v8_linux_dbg': set(['defaulttests']),
       'v8_linux_nodcheck_rel': set(['defaulttests']),
       'v8_linux_gcc_compile_rel': set(['defaulttests']),
       'v8_linux64_rel': set(['defaulttests']),

testing/commit_queue/config.json

@@ -11,6 +11,7 @@
                 "v8_linux_gcc_compile_rel": ["defaulttests"],
                 "v8_linux_nodcheck_rel": ["defaulttests"],
                 "v8_linux_rel": ["defaulttests"],
+                "v8_linux_dbg": ["defaulttests"],
                 "v8_mac_rel": ["defaulttests"],
                 "v8_presubmit": ["defaulttests"],
                 "v8_win64_rel": ["defaulttests"],