Specify --test-machine-name to maps_pixel_test running on bots.

content/test/gpu/gpu_tests/cloud_storage_test_base.py

Index: content/test/gpu/gpu_tests/cloud_storage_test_base.py
diff --git a/content/test/gpu/gpu_tests/cloud_storage_test_base.py b/content/test/gpu/gpu_tests/cloud_storage_test_base.py
index fee9ba2b5f3cfebfffdf641e35251be6009e3d28..3c8d04b4a6677e029443e1860eb7769fa7fc46a9 100644
--- a/content/test/gpu/gpu_tests/cloud_storage_test_base.py
+++ b/content/test/gpu/gpu_tests/cloud_storage_test_base.py
@@ -33,7 +33,10 @@ def _CompareScreenshotSamples(screenshot, expectations, device_pixel_ratio,
     for expectation in expectations:
       if "scale_factor_overrides" in expectation:
         for override in expectation["scale_factor_overrides"]:
-          if override["machine_name"] in test_machine_name:
+          # Require exact match to avoid confusion, because some
+          # machine models and names might be subsets of others
+          # (e.g. Nexus 5 vs Nexus 5X).
+          if override["machine_name"] == test_machine_name:
             logging.warning('Overriding device_pixel_ratio ' +
                             str(device_pixel_ratio) + ' with scale factor ' +
                             str(override["scale_factor"]))