Use Shadow DOM to display fallback content for images

LayoutTests/fast/images/alt-text-wrapping-expected.html

Index: LayoutTests/fast/images/alt-text-wrapping-expected.html
diff --git a/LayoutTests/fast/images/alt-text-wrapping-expected.html b/LayoutTests/fast/images/alt-text-wrapping-expected.html
new file mode 100644
index 0000000000000000000000000000000000000000..52f8d5aa1b4a5d9e4127f326c92769cf0ad0bfe6
--- /dev/null
+++ b/LayoutTests/fast/images/alt-text-wrapping-expected.html
@@ -0,0 +1,17 @@
+<style>
+    #alttext-container {
+        overflow: hidden;
+        border: 1px solid silver;
+        padding: 1px;
+        display: inline-block;        
+        width: 100px;
+        height: 100px;
+        box-sizing: border-box;
+    }                                 
+    #alttext { overflow: hidden; }
+</style>                                    
+<div id="alttext-container">                  
+    <img src="data:image/png;base64,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" width="16" height="16" align="left" style="margin: 0px">
+    <div id="alttext">This text should wrap inside the fallback content.</div>         
+</div>                                                  
+