[Preview Tab] Add BottomSheet to the test config.

Updates the test-config for the Preview Tab (aka Ephemeral Tab) to
use the Bottom-Sheet version instead of the Overlay version.

BUG=913075

Change-Id: I92ad27d5deebbd8cceef87901fac6c50198fe756
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1947952Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Donn Denman <donnd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721360}

testing/variations/fieldtrial_testing_config.json

@@ -2520,7 +2520,8 @@
                 {
                     "name": "Enabled",
                     "enable_features": [
-                        "EphemeralTab"
+                        "EphemeralTab",
+                        "EphemeralTabUsingBottomSheet"
                     ]
                 }
             ]