Enable +/- zoom control for the sub-window.
This should also enable the pinch zoom

Fix http://b/issue?id=2530401
diff --git a/src/com/android/browser/Tab.java b/src/com/android/browser/Tab.java
index 8cbaa4d..e78fa45 100644
--- a/src/com/android/browser/Tab.java
+++ b/src/com/android/browser/Tab.java
@@ -1420,6 +1420,8 @@
             mSubView.setScrollBarStyle(View.SCROLLBARS_OUTSIDE_OVERLAY);
             // use trackball directly
             mSubView.setMapTrackballToArrowKeys(false);
+            // Enable the built-in zoom
+            mSubView.getSettings().setBuiltInZoomControls(true);
             mSubView.setWebViewClient(new SubWindowClient(mWebViewClient));
             mSubView.setWebChromeClient(new SubWindowChromeClient(
                     mWebChromeClient));