Enable CriticlaPersistedTabData in fieldtrial_testing_config

This was determined to be necessary in cl/342061697. This is a reland of
chromium CL 2548503. An explanation was provided in the commit
description why loadNextTab() should be called on the UI thread instead
of restoreTabs(false) on the background thread. I deep dived the issue
created by that CL and found that a policy violation occurs when the
directory for CriticalPersistedTabData is created/read. I change the
policy and revert back after the directory is read (and created if
necessary) like we do in several other parts of TabPersistentStore.

Bug: 1151871
Change-Id: I306d4d2bc2ab2b074621d76d1926cc07d1b56b38
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2574620
Commit-Queue: David Maunder <davidjm@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834421}

chrome/android/java/src/org/chromium/chrome/browser/tabmodel/TabPersistentStore.java

@@ -457,9 +457,7 @@ public class TabPersistentStore extends TabPersister {
         }
 
         // Restore the tabs from the second activity asynchronously.
-        PostTask.postTask(TaskTraits.BEST_EFFORT_MAY_BLOCK, () -> {
-            restoreTabs(false);
-        });
+        loadNextTab();
     }
 
     /**

chrome/browser/tab/java/src/org/chromium/chrome/browser/tab/state/FilePersistedTabDataStorage.java

@@ -29,6 +29,8 @@ import java.io.IOException;
 import java.util.LinkedList;
 import java.util.Locale;
 
+import android.os.StrictMode;
+
 /**
  * {@link PersistedTabDataStorage} which uses a file for the storage
  */
@@ -45,8 +47,13 @@ public class FilePersistedTabDataStorage implements PersistedTabDataStorage {
         private static File sDirectory;
 
         static {
-            sDirectory =
-                    ContextUtils.getApplicationContext().getDir(sBaseDirName, Context.MODE_PRIVATE);
+            StrictMode.ThreadPolicy oldPolicy = StrictMode.allowThreadDiskWrites();
+            try {
+                sDirectory =
+                ContextUtils.getApplicationContext().getDir(sBaseDirName, Context.MODE_PRIVATE);
+            } finally {
+                StrictMode.setThreadPolicy(oldPolicy);
+            }
         }
     }
     private SequencedTaskRunner mSequencedTaskRunner;

testing/variations/fieldtrial_testing_config.json

@@ -2035,6 +2035,21 @@
             ]
         }
     ],
+    "CriticalPersistedTabData": [
+        {
+            "platforms": [
+                "android"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "CriticalPersistedTabData"
+                    ]
+                }
+            ]
+        }
+    ],
     "CrosSchedulerCore": [
         {
             "platforms": [