Let GetPreferredTryMasters read from CQ config.

PRESUBMIT.py

Index: PRESUBMIT.py
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 2bb31b461f6a27b2f8e535f575ce95a50e236fd1..2dfe25ab081ed299df2e03c323bca3526bd8e545 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -344,17 +344,27 @@ def CheckChangeOnCommit(input_api, output_api):
 
 
 def GetPreferredTryMasters(project, change):
-    return {
-        'tryserver.blink': {
-            'android_blink_compile_dbg': set(['defaulttests']),
-            'android_blink_compile_rel': set(['defaulttests']),
-            'android_chromium_gn_compile_rel': set(['defaulttests']),
-            'linux_blink_compile_dbg': set(['defaulttests']),
-            'linux_blink_rel': set(['defaulttests']),
-            'linux_chromium_gn_rel': set(['defaulttests']),
-            'mac_blink_compile_dbg': set(['defaulttests']),
-            'mac_blink_rel': set(['defaulttests']),
-            'win_blink_compile_dbg': set(['defaulttests']),
-            'win_blink_rel': set(['defaulttests']),
-        },
-    }
+    import json
+    import os.path
+    import platform
+    import subprocess
+
+    cq_config_path = os.path.join(
+        change.RepositoryRoot(), 'infra', 'config', 'cq.cfg')
+    # commit_queue.py below is a script in depot_tools directory, which has a
+    # 'builders' command to retrieve a list of CQ builders from the CQ config.
+    is_win = platform.system() == 'Windows'
+    masters = json.loads(subprocess.check_output(
+        ['commit_queue', 'builders', cq_config_path], shell=is_win))
+
+    try_config = {}
+    for master in masters:
+        try_config.setdefault(master, {})
+        for builder in masters[master]:
+            # Do not trigger presubmit builders, since they're likely to fail
+            # (e.g. OWNERS checks before finished code review), and we're
+            # running local presubmit anyway.
+            if 'presubmit' not in builder:
+                try_config[master][builder] = ['defaulttests']
+
+    return try_config