Update prefs API to return an Option<bool>.

This allows for situations where there is no reasonable default
to apply for the pref value e.g. when we are just listing values

components/compositing/constellation.rs

@@ -891,7 +891,7 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
                                    containing_pipeline_id: PipelineId,
                                    subpage_id: SubpageId,
                                    event: MozBrowserEvent) {
-        assert!(prefs::get_pref("dom.mozbrowser.enabled", false));
+        assert!(prefs::get_pref("dom.mozbrowser.enabled").unwrap_or(false));
 
         // Find the script channel for the given parent pipeline,
         // and pass the event to that script task.
@@ -1373,7 +1373,7 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
 
     // https://developer.mozilla.org/en-US/docs/Web/Events/mozbrowserlocationchange
     fn trigger_mozbrowserlocationchange(&self, pipeline_id: PipelineId) {
-        if prefs::get_pref("dom.mozbrowser.enabled", false) {
+        if prefs::get_pref("dom.mozbrowser.enabled").unwrap_or(false) {
             // Work around borrow checker
             let event_info = {
                 let pipeline = self.pipeline(pipeline_id);

components/compositing/pipeline.rs

@@ -269,7 +269,7 @@ impl Pipeline {
     pub fn trigger_mozbrowser_event(&self,
                                      subpage_id: SubpageId,
                                      event: MozBrowserEvent) {
-        assert!(prefs::get_pref("dom.mozbrowser.enabled", false));
+        assert!(prefs::get_pref("dom.mozbrowser.enabled").unwrap_or(false));
 
         let event = ConstellationControlMsg::MozBrowserEvent(self.id,
                                                              subpage_id,

components/script/dom/htmliframeelement.rs

@@ -42,7 +42,7 @@ use url::{Url, UrlParser};
 use util::str::{self, LengthOrPercentageOrAuto};
 
 pub fn mozbrowser_enabled() -> bool {
-    prefs::get_pref("dom.mozbrowser.enabled", false)
+    prefs::get_pref("dom.mozbrowser.enabled").unwrap_or(false)
 }
 
 #[derive(HeapSizeOf)]

components/script/dom/mouseevent.rs

@@ -171,7 +171,7 @@ impl MouseEventMethods for MouseEvent {
     // This returns the same result as current gecko.
     // https://developer.mozilla.org/en-US/docs/Web/API/MouseEvent/which
     fn Which(&self) -> i32 {
-        if prefs::get_pref("dom.mouseevent.which.enabled", false) {
+        if prefs::get_pref("dom.mouseevent.which.enabled").unwrap_or(false) {
             (self.button.get() + 1) as i32
         } else {
             0

components/style/properties.mako.rs

@@ -494,7 +494,7 @@ pub mod longhands {
                 % for value in values[:-1]:
                     "${value}" => {
                         % if value in experimental_values:
-                            if !::util::prefs::get_pref("layout.${value}.enabled", false) {
+                            if !::util::prefs::get_pref("layout.${value}.enabled").unwrap_or(false) {
                                 return Err(())
                             }
                         % endif
@@ -504,7 +504,7 @@ pub mod longhands {
                 % for value in values[-1:]:
                     "${value}" => {
                         % if value in experimental_values:
-                            if !::util::prefs::get_pref("layout.${value}.enabled", false) {
+                            if !::util::prefs::get_pref("layout.${value}.enabled".unwrap_or(false) {
                                 return Err(())
                             }
                         % endif
@@ -5796,7 +5796,7 @@ impl PropertyDeclaration {
                 % if property.derived_from is None:
                     "${property.name}" => {
                         % if property.experimental:
-                            if !::util::prefs::get_pref("${property.experimental}", false) {
+                            if !::util::prefs::get_pref("${property.experimental}").unwrap_or(false) {
                                 return PropertyDeclarationParseResult::ExperimentalProperty
                             }
                         % endif
@@ -5815,7 +5815,7 @@ impl PropertyDeclaration {
             % for shorthand in SHORTHANDS:
                 "${shorthand.name}" => {
                     % if shorthand.experimental:
-                        if !::util::prefs::get_pref("${shorthand.experimental}", false) {
+                        if !::util::prefs::get_pref("${shorthand.experimental}").unwrap_or(false) {
                             return PropertyDeclarationParseResult::ExperimentalProperty
                         }
                     % endif

components/style/stylesheets.rs

@@ -429,7 +429,7 @@ impl<'a, 'b> AtRuleParser for NestedRuleParser<'a, 'b> {
                 Ok(AtRuleType::WithBlock(AtRulePrelude::FontFace))
             },
             "viewport" => {
-                if ::util::prefs::get_pref("layout.viewport.enabled", false) {
+                if ::util::prefs::get_pref("layout.viewport.enabled").unwrap_or(false) {
                     Ok(AtRuleType::WithBlock(AtRulePrelude::Viewport))
                 } else {
                     Err(())

components/util/prefs.rs

@@ -41,8 +41,8 @@ fn read_prefs() -> Result<HashMap<String, bool>, ()> {
     Ok(prefs)
 }
 
-pub fn get_pref(name: &str, default: bool) -> bool {
+pub fn get_pref(name: &str) -> Option<bool> {
-    *PREFS.lock().unwrap().get(name).unwrap_or(&default)
+    PREFS.lock().unwrap().get(name).cloned()
 }
 
 pub fn set_pref(name: &str, value: bool) {