Fix the ClassCastException on opening ClearSyncDataPreferences.

When opening from ClearSyncDataPreferences, the getActivity() call was casted to Preferences, which was
1.) too specific - this code can be reached from at least three different places, two of which (bookmarks menu, recent tabs menu) are not Preferences; the main entry point is through the privacy settings, which is a Preferences activity, which is why this wasn't discovered immediately
2.) not even needed - PreferencesLauncher.createIntentForSettingsPage accepts any Context as an argument, so getActivity() doesn't need to be casted at all

BUG=585714

Review URL: https://codereview.chromium.org/1701753002

Cr-Commit-Position: refs/heads/master@{#375480}

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

@@ -21,7 +21,6 @@
 import org.chromium.base.metrics.RecordUserAction;
 import org.chromium.chrome.R;
 import org.chromium.chrome.browser.preferences.PrefServiceBridge;
-import org.chromium.chrome.browser.preferences.Preferences;
 import org.chromium.chrome.browser.preferences.PreferencesLauncher;
 import org.chromium.chrome.browser.sync.ui.ClearSyncDataPreferences;
 import org.chromium.ui.text.SpanApplier;
@@ -93,10 +92,9 @@ public void onClick(DialogInterface dialog, int which) {
     }
 
     private void showClearSyncDataPreferences() {
-        Preferences prefActivity = (Preferences) getActivity();
-        Intent intent = PreferencesLauncher.createIntentForSettingsPage(prefActivity,
+        Intent intent = PreferencesLauncher.createIntentForSettingsPage(getActivity(),
                 ClearSyncDataPreferences.class.getName());
-        prefActivity.startActivity(intent);
+        startActivity(intent);
 
         // Cancel out of current sign in.
         SigninManager.get(getActivity()).abortSignIn();