chromium_tests: apply chromium.get_env() when running tests

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "tryserver.blink",
       "--builder",
       "linux_blink_rel",
@@ skipping 400 matching lines @@
       "/path/to/tmp/json",
       "--test-results-server",
       "test-results.appspot.com",
       "--build-number",
       "571",
       "--builder-name",
       "linux_blink_rel",
       "--step-name",
       "webkit_tests (with patch)"
     ],
+    "env": {
+      "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
+    },
     "name": "webkit_tests (with patch)",
     "~followup_annotations": [
       "step returned non-zero exit code: 130",
       "@@@STEP_TEXT@<br/>unexpected_flakes:<br/>flake/slow.html<br/>flake/totally-flakey.html<br/>flake/timeout-then-crash.html<br/><br/>unexpected_failures:<br/>bad/totally-bad-probably.html<br/>tricky/totally-maybe-not-awesome.html<br/><br/>Total executed: 9001<br/>@@@",
       "@@@STEP_FAILURE@@@"
     ]
   },
   {
     "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",
       "linux_blink_rel",
       "--gs-bucket",
       "gs://chromium-layout-test-archives"
     ],
+    "env": {
+      "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/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/linux_blink_rel/571/layout-test-results/results.html@@@",
       "@@@STEP_LINK@(zip)@https://storage.googleapis.com/chromium-layout-test-archives/linux_blink_rel/571/layout-test-results.zip@@@",
       "@@@SET_BUILD_PROPERTY@failure_type@\"INVALID_TEST_RESULTS\"@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "import sys; sys.exit(1)"
     ],
+    "env": {
+      "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
+    },
     "name": "webkit_tests",
     "~followup_annotations": [
       "step returned non-zero exit code: 1",
       "@@@STEP_TEXT@TEST RESULTS WERE INVALID@@@",
       "@@@STEP_FAILURE@@@",
       "@@@SET_BUILD_PROPERTY@failure_hash@\"a860d9d78c0340ef1c42d9fa74626fe7b110eeb0\"@@@"
     ]
   },
   {
     "name": "$result",
     "reason": "1 out of 1 aggregated steps failed. Failures: Step('webkit_tests') failed with return_code 1",
     "status_code": 1
   }
 ]