SigninTestUtil: During tear down, wait for AccountHolders to be removed

Without this change, there is a race condition where it's possible that
an auth token gets requested from the native side (via
OAuth2TokenServiceDelegateAndroid) for an account that was actually
already removed, which causes an exception in
FakeAccountManagerDelegate.getAccountHolder.

https://crrev.com/c/1148392 uncovered this, but I believe the race
condition is there independent of that CL (it's just much less likely
to get triggered).

Bug: 856179
Change-Id: I23bdf54aba90b1bbfd64f45963ea32d08b63c852
Reviewed-on: https://chromium-review.googlesource.com/1156688
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579783}

chrome/test/android/javatests/src/org/chromium/chrome/test/util/browser/signin/SigninTestUtil.java

@@ -8,6 +8,7 @@ import android.accounts.Account;
 import android.annotation.SuppressLint;
 import android.app.Instrumentation;
 import android.content.Context;
+import android.support.annotation.WorkerThread;
 
 import org.chromium.base.ContextUtils;
 import org.chromium.base.ThreadUtils;
@@ -44,6 +45,7 @@ public final class SigninTestUtil {
      *
      * This must be called before native is loaded.
      */
+    @WorkerThread
     public static void setUpAuthForTest(Instrumentation instrumentation) {
         assert sContext == null;
         sContext = instrumentation.getTargetContext();
@@ -63,12 +65,11 @@ public final class SigninTestUtil {
     /**
      * Tears down the test authentication environment.
      */
+    @WorkerThread
     public static void tearDownAuthForTest() {
-        ThreadUtils.runOnUiThreadBlocking(() -> {
-            for (AccountHolder accountHolder : sAddedAccounts) {
-                sAccountManager.removeAccountHolderExplicitly(accountHolder);
-            }
-        });
+        for (AccountHolder accountHolder : sAddedAccounts) {
+            sAccountManager.removeAccountHolderBlocking(accountHolder);
+        }
         sAddedAccounts.clear();
         sContext = null;
     }