Update CacheStorageSequenceChromeOS fieldtrial_testing_config.

This CL updates the testing config to match the new trial launch target
of 8 maximum parallel cache operations.

Bug: 960012
Change-Id: I7d7b973ffaf257683a2c8fb5913b375b4bd205d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2343223Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Ben Kelly <wanderview@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797272}

testing/variations/fieldtrial_testing_config.json

@@ -1369,9 +1369,9 @@
             ],
             "experiments": [
                 {
-                    "name": "Enabled10_v3",
+                    "name": "Enabled8_v4",
                     "params": {
-                        "max_shared_ops": "10"
+                        "max_shared_ops": "8"
                     },
                     "enable_features": [
                         "CacheStorageParallelOps",