Make back button work in CaptivePortalLogin app like a web browser.

Users will be using the app to navigate web pages like they are used to in
web browsers, so the back button should act similarly.  There also isn't
an in-app back button so this adds essentially necessary functionality.

bug:16397616
Change-Id: Idd73d4a048d79e56bf981a8f477336f00eaafc3b

packages/CaptivePortalLogin/src/com/android/captiveportallogin/CaptivePortalLoginActivity.java

@@ -96,6 +96,16 @@ public class CaptivePortalLoginActivity extends Activity {
         return true;
     }
 
+    @Override
+    public void onBackPressed() {
+        WebView myWebView = (WebView) findViewById(R.id.webview);
+        if (myWebView.canGoBack()) {
+            myWebView.goBack();
+        } else {
+            super.onBackPressed();
+        }
+    }
+
     @Override
     public boolean onOptionsItemSelected(MenuItem item) {
         int id = item.getItemId();