Bug 1569902 - part 2: Stop using attribute to consider whether a <br> element is a special node for empty last line or not r=m_kato,smaug

Editor creates a `<br>` element to end of a block if last line
of the block is empty because caret should be placed as there is an empty
line.  Such special `<br>` element has `type` attribute whose value is "_moz".
However, adding/removing the attribute is expensive and such hacky attribute
shouldn't be referred nor changed by web apps.

Therefore, this patch makes `HTMLBRElement` take another specific flag whether
it's a special node for empty last line.  For making the meaning clearer,
this patch calls the such `<br>` elements as "padding `<br>` element for
empty last line" insead of "moz-br".  So, this patch also includes a lot of
renaming methods and variables, and modifying related comments.

Note that with this change, `IMEContentObserver` counts the padding `<br>`
element in `<textarea>` because it's inserted before setting the new flag
and setting the flag does not cause DOM tree mutation.  This issue will be
fixed by the following patches.

Differential Revision: https://phabricator.services.mozilla.com/D39858

dom/base/nsXHTMLContentSerializer.cpp

@@ -208,8 +208,6 @@ bool nsXHTMLContentSerializer::SerializeAttributes(
     PushNameSpaceDecl(aTagPrefix, aTagNamespaceURI, aOriginalElement);
   }
 
-  NS_NAMED_LITERAL_STRING(_mozStr, "_moz");
-
   count = aElement->GetAttrCount();
 
   // Now serialize each of the attributes
@@ -252,15 +250,6 @@ bool nsXHTMLContentSerializer::SerializeAttributes(
     bool isJS = false;
 
     if (kNameSpaceID_XHTML == contentNamespaceID) {
-      //
-      // Filter out special case of <br type="_moz"> or <br _moz*>,
-      // used by the editor.  Bug 16988.  Yuck.
-      //
-      if (namespaceID == kNameSpaceID_None && aTagName == nsGkAtoms::br &&
-          attrName == nsGkAtoms::type && StringBeginsWith(valueStr, _mozStr)) {
-        continue;
-      }
-
       if (mIsCopying && mIsFirstChildOfOL && (aTagName == nsGkAtoms::li) &&
           (attrName == nsGkAtoms::value)) {
         // This is handled separately in SerializeLIValueAttribute()