chromium_tests: apply chromium.get_env() when running tests

scripts/slave/recipes/chromium.expected/chromium_webkit_crash.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "chromium.webkit",
       "--builder",
       "WebKit Linux",
@@ skipping 316 matching lines @@
       "/path/to/tmp/json",
       "--test-results-server",
       "test-results.appspot.com",
       "--build-number",
       "571",
       "--builder-name",
       "WebKit Linux",
       "--step-name",
       "webkit_tests"
     ],
+    "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@<br/>Total executed: n/a<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",
       "WebKit Linux",
       "--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/WebKit_Linux/571/layout-test-results/results.html@@@",
       "@@@STEP_LINK@(zip)@https://storage.googleapis.com/chromium-layout-test-archives/WebKit_Linux/571/layout-test-results.zip@@@"
     ]
   },
   {
     "name": "$result",
     "reason": "1 tests failed: ['webkit_tests']",
     "status_code": 1
   }
 ]