Use Shadow DOM to display fallback content for images

LayoutTests/fast/forms/002-expected.txt

Index: LayoutTests/fast/forms/002-expected.txt
diff --git a/LayoutTests/fast/forms/002-expected.txt b/LayoutTests/fast/forms/002-expected.txt
index baacbfa920d68b98bed497e5852cb42286338843..414223e61fc2cf47485444640be86004510a4d50 100644
--- a/LayoutTests/fast/forms/002-expected.txt
+++ b/LayoutTests/fast/forms/002-expected.txt
@@ -4,5 +4,12 @@ layer at (0,0) size 800x600
   RenderBlock {HTML} at (0,0) size 800x600
     RenderBody {BODY} at (8,8) size 784x584
       RenderBlock (floating) {DIV} at (0,0) size 156x24 [border: (2px solid #FF0000)]
-        RenderImage {INPUT} at (2,2) size 152x20
+        RenderBlock {INPUT} at (2,2) size 152x20
         RenderText {#text} at (0,0) size 0x0
+layer at (10,10) size 152x20 clip at (11,11) size 150x18
+  RenderBlock {DIV} at (0,0) size 152x20 [border: (1px solid #C0C0C0)]
+    RenderImage {IMG} at (2,2) size 16x16
+layer at (28,12) size 132x16
+  RenderBlock {DIV} at (18,2) size 132x16
+    RenderText {#text} at (0,0) size 41x16
+      text run at (0,0) width 41: "Submit"