In rebaseline-cl, trigger try jobs with builder per invocation of git cl try.

third_party/WebKit/Tools/Scripts/webkitpy/tool/commands/rebaseline_cl.py

Index: third_party/WebKit/Tools/Scripts/webkitpy/tool/commands/rebaseline_cl.py
diff --git a/third_party/WebKit/Tools/Scripts/webkitpy/tool/commands/rebaseline_cl.py b/third_party/WebKit/Tools/Scripts/webkitpy/tool/commands/rebaseline_cl.py
index 85746dc0d0a0b37a67713be53e84e5acb7adb93b..f988eda41c47dc8cb38cfa775dc4ca883c9a0647 100644
--- a/third_party/WebKit/Tools/Scripts/webkitpy/tool/commands/rebaseline_cl.py
+++ b/third_party/WebKit/Tools/Scripts/webkitpy/tool/commands/rebaseline_cl.py
@@ -105,11 +105,12 @@ class RebaselineCL(AbstractParallelRebaselineCommand):
         for builder in sorted(builders_without_builds):
             _log.info('  %s', builder)
 
-        git_cl = GitCL(self._tool.executive)
-        command = ['try']
+        # If the builders may be under different masters, then they cannot
+        # all be started in one invocation of git cl try without providing
+        # master names. Doing separate invocations is slower, but always works
+        # even when there are builders under different master names.
         for builder in sorted(builders_without_builds):
-            command += ['-b', builder]
-        self.git_cl().run(command)
+            self.git_cl().run(['try', '-b', builder])
 
     def _test_prefix_list(self, issue_number, only_changed_tests):
         """Returns a collection of test, builder and file extensions to get new baselines for.