Add autofill preview support for Textarea

LayoutTests/fast/forms/suggested-value-expected.txt

Index: LayoutTests/fast/forms/suggested-value-expected.txt
diff --git a/LayoutTests/fast/forms/suggested-value-expected.txt b/LayoutTests/fast/forms/suggested-value-expected.txt
index b2f7e6d67691e1db6aa379948aefa9cf62060de7..5bd1f555d18ce50199c73ffd1ea45c4e8997dd2b 100644
--- a/LayoutTests/fast/forms/suggested-value-expected.txt
+++ b/LayoutTests/fast/forms/suggested-value-expected.txt
@@ -1,5 +1,6 @@
-This test setting a suggested value on an input element.
-The dump below should have the "suggested value" instead of "initial value".
+This test setting suggested values on an input element 
+and a textarea element. The dump below should have the "suggested value"
+instead of "initial value".
 | <input>
 |   id="test"
 |   type="text"
@@ -9,7 +10,16 @@ The dump below should have the "suggested value" instead of "initial value".
 |     <div>
 |       id="inner-editor"
 |       "suggested value"
+| <textarea>
+|   id="textarea"
+|   this.value="initial value"
+|   <shadow:root>
+|     <div>
+|       id="inner-editor"
+|       "suggested value"
 | "input.value: initial value"
 | "internals.suggestedValue(input): suggested value"
 | "input.selectionStart: 0"
 | "input.selectionEnd: 0"
+| "textarea.value: initial value"
+| "internals.suggestedValue(textarea): suggested value"