Update a Finch config for ServiceWorkerAggressiveCodeCache

This experiment uses a variation parameter to control how
V8 code cache is generated for service worker scripts.
We need to put the value of the parameter explicitly.

Bug: 915081
Change-Id: Ie9cb565e3ad09396de5d125a46b9b5db755bbb14
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1597494
Auto-Submit: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658374}

testing/variations/fieldtrial_testing_config.json

@@ -4497,6 +4497,9 @@
             "experiments": [
                 {
                     "name": "Enabled_20190318",
+                    "params": {
+                        "sw_cache_strategy": "idletask"
+                    },
                     "enable_features": [
                         "ServiceWorkerAggressiveCodeCache"
                     ]