Update SlowDCTimerInterruptsWin group suffix

This changes the SlowDCTimerInterruptsWin experiment group name from
"Enabled" to "Enabled_20200424", to match the latest group name in
the .gcl configuration file.

The suffix was added to the .gcl file in cl/308344693.

Change-Id: Ie62db49856d15e0bd9d327d217980fc48edb2340
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2166124
Auto-Submit: Jesse McKenna <jessemckenna@google.com>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#763522}

testing/variations/fieldtrial_testing_config.json

@@ -5552,7 +5552,7 @@
             ],
             "experiments": [
                 {
-                    "name": "Enabled",
+                    "name": "Enabled_20200424",
                     "enable_features": [
                         "SlowDCTimerInterruptsWin"
                     ]