Revert "Use ThreadUtils.postOnUiThread in SigninManager.java"

This reverts commit 454a3530.

Reason for revert: As we are going towards layering into
AuthenticationService instead of componentization of
SigninManager.java, there is no need to remove the dependency
on UiThreadTaskTraits anymore.
Design doc regarding the new code structure:
https://docs.google.com/document/d/171FRUtA_FlU2ZIReQL8cZuZwAeBk0wHymN2w2h01eso/edit?usp=sharing

Original change's description:
> Use ThreadUtils.postOnUiThread in SigninManager.java
>
> Use ThreadUtils instead of PostTask and UiThreadTaskTraits. Remove
> content_public dependency. Enabling the componentization of
> SigninManager. As agreed with ksolt@, see crbug.com/981470. Change was
> originally introduced in: crrev.com/c/1432841. Follow up migration will
> be to return to PostTask, but using current thread traits. It is an
> ongoing work for Ksolt@: crrev.com/c/1645836, crbug.com/981470.
>
> Bug: 981470, 963408
> Change-Id: Ia7a0fa54fd022182c973230e3356671c90bbb655
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1687244
> Commit-Queue: Pâris Meuleman <pmeuleman@chromium.org>
> Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
> Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
> Auto-Submit: Pâris Meuleman <pmeuleman@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#675187}

TBR=sdefresne@chromium.org,bsazonov@chromium.org,pmeuleman@chromium.org

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

Bug: 981470, 963408
Change-Id: Iacff2103b5d3c0353b650fabb9a97d72e95f7ceb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1708072
Commit-Queue: Pâris Meuleman <pmeuleman@chromium.org>
Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679988}

chrome/android/java/src/org/chromium/chrome/browser/signin/SigninManager.java

@@ -26,10 +26,12 @@ import org.chromium.base.annotations.JCaller;
 import org.chromium.base.annotations.NativeMethods;
 import org.chromium.base.metrics.RecordHistogram;
 import org.chromium.base.metrics.RecordUserAction;
+import org.chromium.base.task.PostTask;
 import org.chromium.components.signin.AccountIdProvider;
 import org.chromium.components.signin.AccountManagerFacade;
 import org.chromium.components.signin.AccountTrackerService;
 import org.chromium.components.signin.ChromeSigninController;
+import org.chromium.content_public.browser.UiThreadTaskTraits;
 
 import java.util.ArrayList;
 import java.util.List;
@@ -338,8 +340,7 @@ public class SigninManager implements AccountTrackerService.OnSystemAccountsSeed
     }
 
     private void notifySignInAllowedChanged() {
-        // TODO(crbug.com/981470) Use PostTask and CurrentThread traits.
-        ThreadUtils.postOnUiThread(() -> {
+        PostTask.postTask(UiThreadTaskTraits.DEFAULT, () -> {
             for (SignInAllowedObserver observer : mSignInAllowedObservers) {
                 observer.onSignInAllowedChanged();
             }
@@ -523,15 +524,13 @@ public class SigninManager implements AccountTrackerService.OnSystemAccountsSeed
             mCallbacksWaitingForPendingOperation.add(runnable);
             return;
         }
-        // TODO(crbug.com/981470) Use PostTask and CurrentThread traits
-        ThreadUtils.postOnUiThread(runnable);
+        PostTask.postTask(UiThreadTaskTraits.DEFAULT, runnable);
     }
 
     private void notifyCallbacksWaitingForOperation() {
         ThreadUtils.assertOnUiThread();
         for (Runnable callback : mCallbacksWaitingForPendingOperation) {
-            // TODO(crbug.com/981470) Use PostTask and CurrentThread traits.
-            ThreadUtils.postOnUiThread(callback);
+            PostTask.postTask(UiThreadTaskTraits.DEFAULT, callback);
         }
         mCallbacksWaitingForPendingOperation.clear();
     }
@@ -665,8 +664,7 @@ public class SigninManager implements AccountTrackerService.OnSystemAccountsSeed
         assert mSignOutState != null;
 
         if (mSignOutState.mCallback != null) {
-            // TODO(crbug.com/981470) Use PostTask and CurrentThread traits.
-            ThreadUtils.postOnUiThread(mSignOutState.mCallback);
+            PostTask.postTask(UiThreadTaskTraits.DEFAULT, mSignOutState.mCallback);
         }
         mSignOutState = null;
         notifyCallbacksWaitingForOperation();