[Paint Preview] Add PaintPreviewShowOnStartup to fieldtrial config

We are planning to experiment on beta in M87. This sets up the field
trial config so the experiment can be prepared for beta rollout in
finch.

Launch Bug is https://crbug.com/1008520

Bug: 994742
Change-Id: I4bac78b3e65b3f60bb49e1d8167852c2ba9e9855
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2418893
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808456}

testing/variations/fieldtrial_testing_config.json

@@ -5195,6 +5195,21 @@
             ]
         }
     ],
+    "PaintPreviewShowOnStartup": [
+        {
+            "platforms": [
+                "android"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled_20200820",
+                    "enable_features": [
+                        "PaintPreviewShowOnStartup"
+                    ]
+                }
+            ]
+        }
+    ],
     "ParallelDownloadWithQUICAndHTTP2": [
         {
             "platforms": [