[StartSurface] Change to single pane start surface in field trial config

Bug: 1004441
Change-Id: Ia7d65837befbcc88f98c23e7644d6c2739bb0f7d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2028172
Auto-Submit: Ganggui Tang <gogerald@chromium.org>
Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Jesse Doherty <jwd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736612}

testing/variations/fieldtrial_testing_config.json

@@ -1520,7 +1520,7 @@
                 {
                     "name": "Enabled",
                     "params": {
-                        "start_surface_variation": "tasksonly",
+                        "start_surface_variation": "single",
                         "tab_switcher_on_return_time_ms": "3600000"
                     },
                     "enable_features": [