[Android][Signin] Change IdentityManager.Observer methods to default

This CL changes the onPrimaryAccountSet() and onPrimaryAccountCleared()
methods to default in IdentityManager.Observer to avoid redundant
empty implementation.

Bug: 1132873
Change-Id: I573cb7e4e6bb4e4d12217aff1249a9da8f249dd0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2438424Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Commit-Queue: Alice Wang <aliceywang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813302}

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

@@ -32,7 +32,6 @@ import org.chromium.components.browser_ui.util.AvatarGenerator;
 import org.chromium.components.signin.AccountManagerFacadeProvider;
 import org.chromium.components.signin.ProfileDataSource;
 import org.chromium.components.signin.base.AccountInfo;
-import org.chromium.components.signin.base.CoreAccountInfo;
 import org.chromium.components.signin.identitymanager.IdentityManager;
 
 import java.util.ArrayList;
@@ -280,18 +279,6 @@ public class ProfileDataCache implements ProfileDownloader.Observer, ProfileData
         }
     }
 
-    /**
-     * Implements {@link IdentityManager.Observer}.
-     */
-    @Override
-    public void onPrimaryAccountSet(CoreAccountInfo account) {}
-
-    /**
-     * Implements {@link IdentityManager.Observer}.
-     */
-    @Override
-    public void onPrimaryAccountCleared(CoreAccountInfo account) {}
-
     /**
      * Returns a profile data cache object without a badge.The badge is put with respect to
      * R.dimen.user_picture_size. So this method only works with the user avatar of this size.

components/signin/public/android/java/src/org/chromium/components/signin/identitymanager/IdentityManager.java

@@ -32,14 +32,14 @@ public class IdentityManager {
          * Called when an account becomes the user's primary account.
          * This method is not called during a reauth.
          */
-        void onPrimaryAccountSet(CoreAccountInfo account);
+        default void onPrimaryAccountSet(CoreAccountInfo account) {}
 
         /**
          * Called when the user moves from having a primary account to no longer having a primary
          * account (note that the user may still have an *unconsented* primary account after this
          * event).
          */
-        void onPrimaryAccountCleared(CoreAccountInfo account);
+        default void onPrimaryAccountCleared(CoreAccountInfo account) {}
 
         /**
          * Called when the Gaia cookie has been deleted explicitly by a user action, e.g. from