Don't call restoreTabs(false) on background thread

The TabState replacement CriticalPersistedTabData makes the assumption
that it is called from the UI thread. This is not the case in for
restoreTabs(false) which is explicitly called on the background thread
and causes the new Tab persistence system to crash. After a deep dive, I
determined that restoreTabs(false) is equivalent to loadNextTab()
because all the code in restoreTabs is skipped except for loadNextTab()
(since setActiveTab is false). After studying loadNextTab() I found that
the 'heavy lifting code' (e.g saving, deletion) is explicitly called
on the background thread so calling loadNextTab() or restoreTabs(false)
from the background thread is not necessary. Calling loadNextTab or
restoreTabs(false) from the UI thread will not cause a performance
regression as the 'heavy lifting' code is already delegated to the
background thread and in doing so will result in the correct consumption
of CriticalPersistedTabData.

Bug: 1148972
Change-Id: I15cfd90125039cf957b88148881765a59fda4556
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2548503Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: David Maunder <davidjm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829318}

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

@@ -459,9 +459,7 @@ public class TabPersistentStore extends TabPersister {
         }
 
         // Restore the tabs from the second activity asynchronously.
-        PostTask.postTask(TaskTraits.BEST_EFFORT_MAY_BLOCK, () -> {
-            restoreTabs(false);
-        });
+        loadNextTab();
     }
 
     /**

testing/variations/fieldtrial_testing_config.json

@@ -1958,6 +1958,21 @@
             ]
         }
     ],
+    "CriticalPersistedTabData": [
+        {
+            "platforms": [
+                "android"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "CriticalPersistedTabData"
+                    ]
+                }
+            ]
+        }
+    ],
     "CrosSchedulerCore": [
         {
             "platforms": [