Add a UI for setting JS flags.

The UI only shows up when 'about:debug' is typed in the address bar.
JS flags are a string '--heap-stats --log-gc' in V8 etc.

Currently this is only used by V8 build for debugging purpose.
diff --git a/res/xml/debug_preferences.xml b/res/xml/debug_preferences.xml
index 4f55f6b..c39ae5b 100644
--- a/res/xml/debug_preferences.xml
+++ b/res/xml/debug_preferences.xml
@@ -51,9 +51,14 @@
             android:title="@string/pref_development_nav_dump" />
 
         <com.android.browser.BrowserSearchpagePreference
-                android:key="search_url" 
-                android:title="@string/pref_development_search_url"
-                android:singleLine="true" />
+            android:key="search_url"
+            android:title="@string/pref_development_search_url"
+            android:singleLine="true" />
+
+        <EditTextPreference
+            android:key="js_engine_flags"
+            android:title="@string/js_engine_flags"
+            android:singleLine="true" />
 
         <ListPreference
             android:key="user_agent"