Change enclosingTextControl to take const Node ptr

Function enclosingTextControl(Node*) is not supposed to change anything,
and hence, is changed to take a const Node* instead.

BUG=n/a

Review-Url: https://codereview.chromium.org/2561633005
Cr-Commit-Position: refs/heads/master@{#437499}

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

@@ -865,7 +865,7 @@ TextControlElement* enclosingTextControl(const Position& position) {
   return enclosingTextControl(position.computeContainerNode());
 }
 
-TextControlElement* enclosingTextControl(Node* container) {
+TextControlElement* enclosingTextControl(const Node* container) {
   if (!container)
     return nullptr;
   Element* ancestor = container->ownerShadowHost();

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

@@ -223,7 +223,7 @@ inline bool isTextControlElement(const Element& element) {
 DEFINE_HTMLELEMENT_TYPE_CASTS_WITH_FUNCTION(TextControlElement);
 
 TextControlElement* enclosingTextControl(const Position&);
-TextControlElement* enclosingTextControl(Node*);
+TextControlElement* enclosingTextControl(const Node*);
 
 }  // namespace blink