Bug 767672. Enable smooth scrolling on webapp runtime. r=myk

webapprt/prefs.js

@@ -26,3 +26,6 @@ pref("extensions.blocklist.level", 2);
 pref("extensions.blocklist.url", "https://addons.mozilla.org/blocklist/3/%APP_ID%/%APP_VERSION%/%PRODUCT%/%BUILD_ID%/%BUILD_TARGET%/%LOCALE%/%CHANNEL%/%OS_VERSION%/%DISTRIBUTION%/%DISTRIBUTION_VERSION%/%PING_COUNT%/%TOTAL_PING_COUNT%/%DAYS_SINCE_LAST_PING%/");
 pref("extensions.blocklist.detailsURL", "https://www.mozilla.com/%LOCALE%/blocklist/");
 pref("extensions.blocklist.itemURL", "https://addons.mozilla.org/%LOCALE%/%APP%/blocked/%blockID%");
+
+// Enable smooth scrolling
+pref("general.smoothScroll", true);