Make _merge_isolated_script_shards robust to harness failures.

scripts/slave/recipes/chromium.py

Index: scripts/slave/recipes/chromium.py
diff --git a/scripts/slave/recipes/chromium.py b/scripts/slave/recipes/chromium.py
index ec72f177b3c355a87c37fa1539d229fb3a1e73fb..e6f62bd1e61998634945ec6b7250d72e25ecdb51 100644
--- a/scripts/slave/recipes/chromium.py
+++ b/scripts/slave/recipes/chromium.py
@@ -592,6 +592,41 @@ def GenTests(api):
   )
 
   yield (
+    api.test('dynamic_swarmed_sharded_isolated_script_test_harness_failure') +
+    api.properties.generic(mastername='chromium.linux',
+                           buildername='Linux Tests',
+                           parent_buildername='Linux Builder') +
+    api.properties(swarm_hashes={
+      'telemetry_gpu_unittests': 'eeeeeeeeeeeeeeeeeeeeeeeeeeeeee',
+    }) +
+    api.platform('linux', 64) +
+    api.override_step_data(
+        'read test spec (chromium.linux.json)',
+        api.json.output({
+            'Linux Tests': {
+                'isolated_scripts': [
+                    {
+                        'isolate_name': 'telemetry_gpu_unittests',
+                        'name': 'telemetry_gpu_unittests',
+                        'swarming': {
+                            'can_use_on_swarming_builders': True,
+                            'shards': 2,
+                        },
+                    },
+                ],
+            },
+        })
+    ) +
+    api.override_step_data(
+      'telemetry_gpu_unittests on Ubuntu-12.04',
+      api.test_utils.canned_isolated_script_output(
+        passing=True, is_win=False, swarming=True,
+        shards=2, isolated_script_passing=True, valid=True,
+        empty_shards=[1]),
+      retcode=1)
+  )
+
+  yield (
     api.test('dynamic_swarmed_isolated_script_test_linux_gpu') +
     api.properties.generic(mastername='chromium.linux',
                            buildername='Linux Tests',