[Paint Holding] Enable in fieldtrial_testing_config

This unblocks moving the study to Beta.

Bug: 1120345
Change-Id: I9c74480ae41d45aa4d1647f5848367ff89bd0a42
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2586122Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Stephen Chenney <schenney@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836712}

testing/variations/fieldtrial_testing_config.json

@@ -5288,7 +5288,7 @@
             ],
             "experiments": [
                 {
-                    "name": "Disabled",
+                    "name": "Enabled_20201211",
                     "disable_features": [
                         "PaintHoldingCrossOrigin"
                     ]