timeout_util: log when timeouts were due to master deadline

BUG=chromium:593089
TEST=unit tests

Change-Id: I753ea798fec1e5d1a60044f10834e26123a54b61
Reviewed-on: https://chromium-review.googlesource.com/331680
Commit-Ready: Aviv Keshet <akeshet@chromium.org>
Tested-by: Aviv Keshet <akeshet@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index ffd329b..7cd1331 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -1245,6 +1245,7 @@
     _SetupConnections(options, build_config)
     retry_stats.SetupStats()
 
+    timeout_display_message = None
     # For master-slave builds: Update slave's timeout using master's published
     # deadline.
     if options.buildbot and options.master_build_id is not None:
@@ -1263,11 +1264,14 @@
           logging.info('Updating slave build timeout to %d seconds enforced '
                        'by the master', slave_timeout)
           options.timeout = slave_timeout
+          timeout_display_message = ('Slave reached the timeout deadline set '
+                                     'by master.')
       else:
         logging.warning('Could not get master deadline for master-slave build. '
                         'Can not set slave timeout.')
 
     if options.timeout > 0:
-      stack.Add(timeout_util.FatalTimeout, options.timeout)
+      stack.Add(timeout_util.FatalTimeout, options.timeout,
+                timeout_display_message)
 
     _RunBuildStagesWrapper(options, site_config, build_config)