Add an option to forceably override user-scalable

Change-Id: I48e0fb0c3472ee51fc907f5b0d307a64400cfe36
diff --git a/src/com/android/browser/PreferenceKeys.java b/src/com/android/browser/PreferenceKeys.java
index 929339f..7976869 100644
--- a/src/com/android/browser/PreferenceKeys.java
+++ b/src/com/android/browser/PreferenceKeys.java
@@ -26,6 +26,7 @@
     // ----------------------
     static final String PREF_MIN_FONT_SIZE = "min_font_size";
     static final String PREF_TEXT_SIZE = "text_size";
+    static final String PREF_FORCE_USERSCALABLE = "force_userscalable";
 
     // ----------------------
     // Keys for advanced_preferences.xml