Add debug option to disable performance features for testing

Change-Id: I508448907a83fb32028cb28eb64b72f97a863fd0
diff --git a/src/com/android/browser/PreferenceKeys.java b/src/com/android/browser/PreferenceKeys.java
index 0cc635a..2f144c5 100644
--- a/src/com/android/browser/PreferenceKeys.java
+++ b/src/com/android/browser/PreferenceKeys.java
@@ -53,6 +53,7 @@
     // ----------------------
     static final String PREF_ENABLE_HARDWARE_ACCEL = "enable_hardware_accel";
     static final String PREF_ENABLE_HARDWARE_ACCEL_SKIA = "enable_hardware_accel_skia";
+    static final String PREF_DISABLE_PERF = "disable_perf";
 
     // ----------------------
     // Keys for general_preferences.xml