[Editing] Use VisiblePosition stardOfWord for textarea in spell checker.

third_party/WebKit/Source/core/html/TextControlElement.cpp

Index: third_party/WebKit/Source/core/html/TextControlElement.cpp
diff --git a/third_party/WebKit/Source/core/html/TextControlElement.cpp b/third_party/WebKit/Source/core/html/TextControlElement.cpp
index 309427990f1212e56ab1413748eca60b3f9a0132..f901aec61b6deaf68ccdb13351fcd86b7218aa54 100644
--- a/third_party/WebKit/Source/core/html/TextControlElement.cpp
+++ b/third_party/WebKit/Source/core/html/TextControlElement.cpp
@@ -870,11 +870,20 @@ TextControlElement* enclosingTextControl(Node* container) {
   if (!container)
     return nullptr;
   Element* ancestor = container->ownerShadowHost();
-  return ancestor && isTextControlElement(*ancestor) &&
-                 container->containingShadowRoot()->type() ==
-                     ShadowRootType::UserAgent
-             ? toTextControlElement(ancestor)
-             : 0;
+
+  if (!ancestor || !isTextControlElement(*ancestor))
+    return nullptr;
+
+  // textarea.shadowRoot is considered not enclosed.
+  if (container->isShadowRoot() &&
+      toShadowRoot(container)->type() == ShadowRootType::UserAgent &&
+      &toShadowRoot(container)->host() == ancestor)
+    return nullptr;
+
+  if (container->containingShadowRoot()->type() != ShadowRootType::UserAgent)
+    return nullptr;
+
+  return toTextControlElement(ancestor);
 }
 
 String TextControlElement::directionForFormData() const {