Add text field change handling for autofill preview in textarea element.

public/web/WebFormControlElement.h

Index: public/web/WebFormControlElement.h
diff --git a/public/web/WebFormControlElement.h b/public/web/WebFormControlElement.h
index a6abd72ffa558f0134e195624207a89dbe14756a..32537f762af6eb1d16625526750c6df33b8bb5b6 100644
--- a/public/web/WebFormControlElement.h
+++ b/public/web/WebFormControlElement.h
@@ -64,6 +64,13 @@ public:
     BLINK_EXPORT bool isAutofilled() const;
     BLINK_EXPORT void setAutofilled(bool);
 
+    // This returns the non-sanitized, exact value inside the text input field
+    // or insisde the textarea.
+    BLINK_EXPORT WebString editingValue() const;
+
+    BLINK_EXPORT int selectionStart() const;
+    BLINK_EXPORT int selectionEnd() const;
+
     // Returns the name that should be used for the specified |element| when
     // storing autofill data.  This is either the field name or its id, an empty
     // string if it has no name and no id.