[Signin] Change method goIncognitoMode in account picker to default

This CL changes the method
AccountPickerCoordniator.Listener#goIncognito to default as it is not
needed for all implementations of the listener. The non necessary
empty implementations are also removed.

Bug: 1093795
Change-Id: I5882d9c35294d2b9de9a59d1e76e5673df7250ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2317799
Commit-Queue: Boris Sazonov <bsazonov@chromium.org>
Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791866}

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

@@ -520,9 +520,6 @@ public abstract class SigninFragmentBase
                 });
     }
 
-    @Override
-    public void goIncognitoMode() {}
-
     @Override
     public void onActivityResult(int requestCode, int resultCode, Intent data) {
         if (requestCode == ADD_ACCOUNT_REQUEST_CODE && resultCode == Activity.RESULT_OK) {

chrome/android/java/src/org/chromium/chrome/browser/signin/account_picker/AccountPickerCoordinator.java

@@ -39,7 +39,7 @@ public class AccountPickerCoordinator {
         /**
          * Notifies when the user clicked the "Go incognito mode" button.
          */
-        void goIncognitoMode();
+        default void goIncognitoMode() {}
     }
 
     private final AccountPickerMediator mMediator;

chrome/android/javatests/src/org/chromium/chrome/browser/signin/DummyAccountPickerTargetFragment.java

@@ -17,7 +17,4 @@ class DummyAccountPickerTargetFragment
 
     @Override
     public void addAccount() {}
-
-    @Override
-    public void goIncognitoMode() {}
 }