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..0e99e3e06d5dfbf9e3d9751bac7e2780918fc4dc
--- /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,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAIAAACQkWg2AAABO0lEQVR4XpWRTytEYRTG3/tHLGQrNfEBpKQkmc8gG0ulxpRsfQ0TTXbI3kZ2ys4tsyBZjN1N8idlYUYaxJzzOKf7ztG1MfN0ens69/x6n/eeYPuo7npS9bCOriXDMQOCTVbO3X+6Wp9mICZWAHqquE1qAA8EQRiFcmYMsQKsAHM2DeKV4qj48ZHB++bHwcXTQ+Mz6u/rABwSIQO0wKW5wuLU8E5yWzlJS7OFvaUJMGelAFkkYici1NIXB4DQbH1JY2ggVoD4N1Kb2AAA149vUmLLxbHaTaNynIJggAz7G7jTyjB5wOv79/LupT47Cu0rcT6S/ai755YUiMM4cgw4/I3ERAaI3y/PiFnYPHOBtfORLKX38AaBERaJbHFeAOY3Eu8ZOYByi7Pl+hzWN+nwWjVB15LhYHXr1PWiH0AYTw4BBjO9AAAAAElFTkSuQmCC" width="16" height="16" align="left" style="margin: 0px">
+    <div id="alttext">This text should wrap inside the fallback content.</div>         
+</div>                                                  
+