[C] use defaultValueCreator to set Frame default Padding (#943)

* [C] use defaultValueCreator to set Frame default Padding

* [docs] update docs

* change to internal

* fix docs

Xamarin.Forms.Core.UnitTests/FrameUnitTests.cs

@@ -287,5 +287,20 @@ namespace Xamarin.Forms.Core.UnitTests
 
 			Assert.AreEqual (new Rectangle (80, 20, 100, 160), child.Bounds);
 		}
-	}	
-}
+
+		[Test]
+		public void SettingPaddingThroughStyle()
+		{
+			var frame = new Frame {
+				Style = new Style(typeof(Frame)) {
+					Setters = {
+						new Setter {Property = Layout.PaddingProperty, Value = 0}
+					}
+				}
+			};
+
+			Assert.AreEqual(new Thickness(0), frame.Padding);
+
+		}
+	}
+}

Xamarin.Forms.Core/Frame.cs

@@ -11,18 +11,21 @@ namespace Xamarin.Forms
 
 		public static readonly BindableProperty HasShadowProperty = BindableProperty.Create("HasShadow", typeof(bool), typeof(Frame), true);
 
-		public static readonly BindableProperty CornerRadiusProperty =
-			BindableProperty.Create(nameof(CornerRadius), typeof(float), typeof(Frame), -1.0f,
+		public static readonly BindableProperty CornerRadiusProperty = BindableProperty.Create(nameof(CornerRadius), typeof(float), typeof(Frame), -1.0f,
 									validateValue: (bindable, value) => ((float)value) == -1.0f || ((float)value) >= 0f);
 
 		readonly Lazy<PlatformConfigurationRegistry<Frame>> _platformConfigurationRegistry;
 
 		public Frame()
 		{
-			Padding = new Size(20, 20);
 			_platformConfigurationRegistry = new Lazy<PlatformConfigurationRegistry<Frame>>(() => new PlatformConfigurationRegistry<Frame>(this));
 		}
 
+		internal override Thickness CreateDefaultPadding()
+		{
+			return 20d;
+		}
+
 		public bool HasShadow
 		{
 			get { return (bool)GetValue(HasShadowProperty); }

Xamarin.Forms.Core/Layout.cs

@@ -54,11 +54,11 @@ namespace Xamarin.Forms
 	{
 		public static readonly BindableProperty IsClippedToBoundsProperty = BindableProperty.Create("IsClippedToBounds", typeof(bool), typeof(Layout), false);
 
-		public static readonly BindableProperty PaddingProperty = BindableProperty.Create("Padding", typeof(Thickness), typeof(Layout), default(Thickness), propertyChanged: (bindable, old, newValue) =>
-		{
-			var layout = (Layout)bindable;
-			layout.UpdateChildrenLayout();
-		});
+		public static readonly BindableProperty PaddingProperty = BindableProperty.Create("Padding", typeof(Thickness), typeof(Layout), default(Thickness),
+									propertyChanged: (bindable, old, newValue) => {
+										var layout = (Layout)bindable;
+										layout.UpdateChildrenLayout();
+									}, defaultValueCreator: (bindable) => ((Layout)bindable).CreateDefaultPadding());
 
 		static IList<KeyValuePair<Layout, int>> s_resolutionList = new List<KeyValuePair<Layout, int>>();
 		static bool s_relayoutInProgress;
@@ -90,6 +90,11 @@ namespace Xamarin.Forms
 			set { SetValue(PaddingProperty, value); }
 		}
 
+		internal virtual Thickness CreateDefaultPadding()
+		{
+			return default(Thickness);
+		}
+
 		internal ObservableCollection<Element> InternalChildren { get; } = new ObservableCollection<Element>();
 
 		internal override ReadOnlyCollection<Element> LogicalChildrenInternal