Bug 1573992 - Convert android.widget_paints_background to static pref. r=njn

Converts android.widget_paints_background to a static pref. From my understanding, this pref only matters on android, and the overridden function in nsIWidget defaults to `false`. Based on that I replaced the entire WidgetPaintsBackground() function with a static pref.

Differential Revision: https://phabricator.services.mozilla.com/D42395

--HG--
extra : moz-landing-system : lando

modules/libpref/init/StaticPrefList.yaml

@@ -178,6 +178,19 @@
   value: false
   mirror: always
 
+#ifdef ANDROID
+  #---------------------------------------------------------------------------
+  # Prefs starting with "android."
+  #---------------------------------------------------------------------------
+
+  # On Android, we want an opaque background to be visible under the page,
+  # so layout should not force a default background.
+-   name: android.widget_paints_background
+    type: bool
+    value: true
+    mirror: always
+#endif
+
 #---------------------------------------------------------------------------
 # Prefs starting with "apz."
 # The apz prefs are explained in AsyncPanZoomController.cpp

modules/libpref/moz.build

@@ -81,7 +81,8 @@ pref_groups = [
 ]
 if CONFIG['OS_TARGET'] == 'Android':
     pref_groups += [
-        'consoleservice'
+        'android',
+        'consoleservice',
     ]
 if CONFIG['FUZZING']:
     pref_groups += [

widget/android/nsWindow.cpp

@@ -15,6 +15,7 @@
 #include "mozilla/MouseEvents.h"
 #include "mozilla/Preferences.h"
 #include "mozilla/RWLock.h"
+#include "mozilla/StaticPrefs_android.h"
 #include "mozilla/StaticPrefs_ui.h"
 #include "mozilla/TouchEvents.h"
 #include "mozilla/TypeTraits.h"
@@ -2189,16 +2190,7 @@ nsresult nsWindow::SynthesizeNativeMouseMove(LayoutDeviceIntPoint aPoint,
 }
 
 bool nsWindow::WidgetPaintsBackground() {
-  static bool sWidgetPaintsBackground = true;
-  static bool sWidgetPaintsBackgroundPrefCached = false;
-
-  if (!sWidgetPaintsBackgroundPrefCached) {
-    sWidgetPaintsBackgroundPrefCached = true;
-    mozilla::Preferences::AddBoolVarCache(
-        &sWidgetPaintsBackground, "android.widget_paints_background", true);
-  }
-
-  return sWidgetPaintsBackground;
+  return StaticPrefs::android_widget_paints_background();
 }
 
 bool nsWindow::NeedsPaint() {