Bug 1623858: part 2) Declare `nsIContent::HasIndependentSelection` `const`. r=smaug

Depends on D67599

Differential Revision: https://phabricator.services.mozilla.com/D67600

--HG--
extra : moz-landing-system : lando

dom/base/FragmentOrElement.cpp

@@ -226,7 +226,7 @@ nsIContent::IMEState nsIContent::GetDesiredIMEState() {
   return state;
 }
 
-bool nsIContent::HasIndependentSelection() {
+bool nsIContent::HasIndependentSelection() const {
   nsIFrame* frame = GetPrimaryFrame();
   return (frame && frame->GetStateBits() & NS_FRAME_INDEPENDENT_SELECTION);
 }

dom/base/nsIContent.h

@@ -622,7 +622,7 @@ class nsIContent : public nsINode {
    * If this content has independent selection, e.g., if this is input field
    * or textarea, this return TRUE.  Otherwise, false.
    */
-  bool HasIndependentSelection();
+  bool HasIndependentSelection() const;
 
   /**
    * If the content is a part of HTML editor, this returns editing