Update ThrottleInstallingServiceWorker fieldtrial_testing_config.

This CL updates the testing config to match the new trial launch
target of 3 maximum simultaneous fetches.

Bug: 1035448
Change-Id: Ibaf8e219709f00b39361ccfa30adc6b4d7b51517
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2343316Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Ben Kelly <wanderview@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796400}

testing/variations/fieldtrial_testing_config.json

@@ -7235,9 +7235,9 @@
             ],
             "experiments": [
                 {
-                    "name": "Emabled5_v1",
+                    "name": "Enabled3_v1",
                     "params": {
-                        "limit": "5"
+                        "limit": "3"
                     },
                     "enable_features": [
                         "ThrottleInstallingServiceWorker"