Created the dialog offering the user to merge their account data or keep it

chrome/android/java/src/org/chromium/chrome/browser/sync/SyncAccountSwitcher.java

Index: chrome/android/java/src/org/chromium/chrome/browser/sync/SyncAccountSwitcher.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/sync/SyncAccountSwitcher.java b/chrome/android/java/src/org/chromium/chrome/browser/sync/SyncAccountSwitcher.java
new file mode 100644
index 0000000000000000000000000000000000000000..8b2c200d1e0b4f2aa19db1d1a0db6ef501cfb991
--- /dev/null
+++ b/chrome/android/java/src/org/chromium/chrome/browser/sync/SyncAccountSwitcher.java
@@ -0,0 +1,85 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package org.chromium.chrome.browser.sync;
+
+import android.app.FragmentManager;
+import android.content.Context;
+import android.preference.Preference;
+import android.preference.Preference.OnPreferenceChangeListener;
+import android.text.TextUtils;
+
+import org.chromium.chrome.browser.preferences.SyncedAccountPreference;
+import org.chromium.chrome.browser.signin.SigninManager;
+import org.chromium.chrome.browser.signin.SigninManager.SignInCallback;
+import org.chromium.chrome.browser.sync.ui.ConfirmImportSyncDataDialog;
+import org.chromium.chrome.browser.sync.ui.ConfirmImportSyncDataDialog.ImportSyncType;
+
+/**
+ * A class that encapsulates the control flow of listeners and callbacks when switching sync
+ * accounts.
+ *
+ * Control flows through the method in this order:
+ *   {@link OnPreferenceChangeListener#onPreferenceChange}
+ *   {@link ConfirmImportSyncDataDialog.Listener#onConfirm}
+ *   {@link SignInCallback#onSignInComplete}
+ */
+public class SyncAccountSwitcher
+        implements OnPreferenceChangeListener, ConfirmImportSyncDataDialog.Listener,
+                   SignInCallback {
+    private static final String TAG = "SyncAccountSwitcher";
+
+    private final SyncedAccountPreference mSyncedAccountPreference;
+    private final Context mContext;
+    private final FragmentManager mFragmentManager;
+
+    private String mNewAccountName;
+
+    /**
+     * Sets up a SyncAccountSwitcher to be ready to accept callbacks.
+     * @param context Context used during signing the user in.
+     * @param fragmentManager To display the data sync fragment.
+     * @param syncedAccountPreference The preference to update once signin has been completed.
+     */
+    public SyncAccountSwitcher(Context context, FragmentManager fragmentManager,
+            SyncedAccountPreference syncedAccountPreference) {
+        mContext = context;
+        mFragmentManager = fragmentManager;
+        mSyncedAccountPreference = syncedAccountPreference;
+    }
+
+    @Override
+    public boolean onPreferenceChange(Preference p, Object newValue) {
+        if (newValue == null) return false;
+
+        mNewAccountName = (String) newValue;
+        String currentAccount = mSyncedAccountPreference.getValue();
+
+        if (TextUtils.equals(mNewAccountName, currentAccount)) return false;
+
+        ConfirmImportSyncDataDialog.showNewInstance(currentAccount, mNewAccountName,
+                ImportSyncType.SWITCHING_SYNC_ACCOUNTS, mFragmentManager, this);
+
+        // Don't update the selected account in the preference. It will be updated by
+        // the call to mSyncAccountListPreference.update() if everything succeeds.
+        return false;
+    }
+
+    @Override
+    public void onConfirm() {
+        assert mNewAccountName != null;
+        SigninManager.get(mContext).signIn(mNewAccountName, null, this);

[newt (away), 2016/02/25 18:01:32]

we want to pass non-null here to get interactive sign-in, right?

[PEConn, 2016/02/25 20:45:12]

Done.

+    }
+
+    @Override
+    public void onSignInComplete() {
+        // Update the Preference so it displays the correct account name.
+        mSyncedAccountPreference.update();
+    }
+
+    @Override
+    public void onSignInAborted() {
+        assert false : "Should not be able to abort sign in when switching sync accounts.";
+    }
+}