That's because of a missing dependency, e.g the compile step didn't build the necessary target


https://luci-logdog.appspot.com/v/?s=chromium%2Fbb%2Ftryserver.chromium.linux%2Flinux_chromium_rel_ng%2F449879%2F%2B%2Frecipes%2Fsteps%2Fisolate_tests%2F0%2Fstdout
---
headless_browsertests  lstat /b/c/b/linux/src/out/Release/headless_browser_tests.pak: no such file or directory
devtools_closure_compile  lstat /b/c/b/linux/src/out/Release/headless_browser_tests.pak: no such file or directory
blink_heap_unittests  lstat /b/c/b/linux/src/out/Release/headless_browser_tests.pak: no such file or directory
content_shell_crash_test  lstat /b/c/b/linux/src/out/Release/headless_browser_tests.pak: no such file or directory
---

yet if you look at:

https://luci-logdog.appspot.com/v/?s=chromium%2Fbb%2Ftryserver.chromium.linux%2Flinux_chromium_rel_ng%2F449879%2F%2B%2Frecipes%2Fsteps%2Fcompile__with_patch_%2F0%2Fstdout
---
/b/rr/tmpgwNB13/rw/checkout/scripts/slave/.recipe_deps/depot_tools/ninja -w 'dupbuild=err' -C /b/c/b/linux/src/out/Release -j 50 app_shell_unittests blink_heap_unittests browser_tests chrome chrome/test:browser_tests chrome/test:chrome_app_unittests chrome/test:interactive_ui_tests chrome/test:performance_browser_tests chrome/test:sync_integration_tests chrome/test:sync_performance_tests chrome/test:unit_tests chrome/tools/service_discovery_sniffer:service_discovery_sniffer chrome:chrome_initial chrome_app_unittests components/constrained_window:unit_tests components/metrics:metrics_unittests components/network_hints/browser:browser components/offline_pages/content/background_loader:test_support components/password_manager/content/renderer:browser_tests components/rappor:rappor_unittests components/safe_browsing_db:database_manager components/safe_browsing_db:remote_database_manager components/safe_browsing_db:safe_browsing_api_handler components/safe_browsing_db:unit_tests_mobile components/safe_browsing_db:v4_database_unittest components/safe_browsing_db:v4_get_hash_protocol_manager_unittest components/safe_browsing_db:v4_local_database_manager_unittest components/sessions:unit_tests components/startup_metric_utils/browser:host components:components_browsertests components:components_perftests components:components_unittests components_browsertests components_unittests content/public/app:both content/shell:content_shell content/test/fuzzer:clear_site_data_fuzzer content/test/fuzzer:origin_trial_token_fuzzer content/test/fuzzer:renderer_fuzzer content/test/fuzzer:renderer_proto_tree_fuzzer content/test/fuzzer:renderer_tree_fuzzer content/test:content_browsertests content/test:content_perftests content/test:content_unittests content_browsertests content_shell_crash_test content_unittests devtools_closure_compile devtools_eslint extensions/shell:app_shell extensions/shell:app_shell_unittests extensions:extensions_browsertests extensions:extensions_unittests extensions_browsertests extensions_unittests headless:embed_resources headless:headless_browsertests headless:headless_example headless:headless_lib headless:headless_shell headless:headless_unittests headless_browsertests headless_unittests interactive_ui_tests mash/browser:browser mash/browser:browser__unused_resources_target mash/webtest:webtest mash/webtest:webtest__unused_resources_target navigation_unittests services/navigation:navigation services/navigation:navigation_unittests sync_integration_tests telemetry_gpu_integration_test telemetry_perf_unittests telemetry_unittests third_party/WebKit/Source/bindings/core/v8:v8_serialized_script_value_fuzzer third_party/WebKit/Source/core:content_security_policy_fuzzer third_party/WebKit/Source/core:html_preload_scanner_fuzzer third_party/WebKit/Source/core:stylesheet_contents_fuzzer third_party/WebKit/Source/core:text_resource_decoder_fuzzer third_party/WebKit/Source/platform/heap:blink_heap_unittests third_party/WebKit/Source/platform:blink_json_parser_fuzzer third_party/WebKit/Source/platform:blink_png_decoder_fuzzer third_party/WebKit/Source/platform:blink_text_codec_UTF_8_fuzzer third_party/WebKit/Source/platform:blink_text_codec_WINDOWS_1252_fuzzer third_party/WebKit/Source/platform:feature_policy_fuzzer third_party/WebKit/Source/platform:mhtml_parser_fuzzer third_party/WebKit/Source/platform:web_icon_sizes_fuzzer third_party/WebKit/Source/web:webkit_unit_tests tools/ipc_fuzzer/fuzzer:ipc_fuzzer tools/ipc_fuzzer/message_dump:ipc_message_dump tools/ipc_fuzzer/message_replay:ipc_fuzzer_replay tools/ipc_fuzzer/message_tools:ipc_message_list tools/ipc_fuzzer/message_tools:ipc_message_util ui/views/examples:views_examples_with_content_exe unit_tests webkit_layout_tests_exparchive webkit_unit_tests
---

so there's a missing dependency.

It's not a swarming issue, not a trooper issue, it's a build sheriff issue.

The dependency error hasn't surfaced in the past few hours on the run list. Bumping down priority but keeping this open for a bit in case it resurfaces.