[test] Bump js-test strings timeout even more

NOTRY=true
TBR=adamk, caitp

Review-Url: https://codereview.chromium.org/2419433006
Cr-Commit-Position: refs/heads/master@{#40257}

test/js-perf-test/JSTests.json

@@ -119,7 +119,7 @@
       "results_regexp": "^%s\\-Strings\\(Score\\): (.+)$",
       "run_count": 1,
       "timeout": 240,
-      "timeout_arm": 360,
+      "timeout_arm": 420,
       "tests": [
         {"name": "StringFunctions"}
       ]