Revert "Don't call restoreTabs(false) on background thread"

This reverts commit e9391ceb.

Reason for revert: causes failures on Lollipop Phone Tester https://crbug.com/1151882

Original change's description:
> 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/+/2548503
> Reviewed-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}

TBR=mpearson@chromium.org,nyquist@chromium.org,dtrainor@chromium.org,davidjm@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 1148972
Change-Id: I0542f6655a1e592d3d070462d5a02734b660a299
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2553425Reviewed-by: Alex Ilin <alexilin@chromium.org>
Commit-Queue: Alex Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830159}

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

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

testing/variations/fieldtrial_testing_config.json

@@ -1956,21 +1956,6 @@
             ]
         }
     ],
-    "CriticalPersistedTabData": [
-        {
-            "platforms": [
-                "android"
-            ],
-            "experiments": [
-                {
-                    "name": "Enabled",
-                    "enable_features": [
-                        "CriticalPersistedTabData"
-                    ]
-                }
-            ]
-        }
-    ],
     "CrosSchedulerCore": [
         {
             "platforms": [