chromium_tests: apply chromium.get_env() when running tests

scripts/slave/recipes/chromium_trybot.expected/full_tryserver_blink_win_blink_rel.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\slave\\kill_processes.py"
     ],
     "name": "taskkill"
   },
   {
@@ skipping 404 matching lines @@
       "--build-number",
       "571",
       "--builder-name",
       "win_blink_rel",
       "--step-name",
       "webkit_tests (with patch)",
       "--master-name",
       "tryserver.blink",
       "--debug-rwt-logging"
     ],
+    "env": {
+      "GOMA_SERVICE_ACCOUNT_JSON_FILE": "C:\\creds\\service_accounts\\service-account-goma-client.json"
+    },
     "name": "webkit_tests (with patch)",
     "~followup_annotations": [
       "@@@STEP_TEXT@<br/>Total executed: 9001<br/>@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]\\scripts\\slave\\chromium\\archive_layout_test_results.py",
       "--results-dir",
       "[SLAVE_BUILD]\\layout-test-results",
       "--build-dir",
       "[SLAVE_BUILD]\\src\\out",
       "--build-number",
       "571",
       "--builder-name",
       "win_blink_rel",
       "--gs-bucket",
       "gs://chromium-layout-test-archives"
     ],
+    "env": {
+      "GOMA_SERVICE_ACCOUNT_JSON_FILE": "C:\\creds\\service_accounts\\service-account-goma-client.json"
+    },
     "name": "archive_webkit_tests_results",
     "~followup_annotations": [
       "@@@STEP_LINK@layout_test_results@https://storage.googleapis.com/chromium-layout-test-archives/win_blink_rel/571/layout-test-results/results.html@@@",
       "@@@STEP_LINK@(zip)@https://storage.googleapis.com/chromium-layout-test-archives/win_blink_rel/571/layout-test-results.zip@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\slave\\process_dumps.py",
       "--target",
       "Release"
     ],
+    "env": {
+      "GOMA_SERVICE_ACCOUNT_JSON_FILE": "C:\\creds\\service_accounts\\service-account-goma-client.json"
+    },
     "name": "process_dumps"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]